Although the title sounds a bit fancy...
This is actually a really easy dish to make. What's a daikon? It's just a big ole' white radish that looks like an overgrown fat carrot. Also known as a mooli. If you cant get hold of one you could actually just use your favourite radishes instead, because the flavours are quite similar.
Mizuna? You can just use your favourite salad leaves - watercress, wild rocket, whatever you fancy.
And the best thing is, you're just literally searing the outside of the tuna, so as long as you get the pan nice and hot, and don't mess about with the tuna, you really can't go wrong with cooking it.
See, it's deceptively simple!
And extremely tasty, too! And if you're entertaining you can make it look all fancy by serving with rice put in a little oiled mould with a strip of Nori (seaweed) in it, to go around the rice and give you some contrast (I served it with sushi rice, I'll be posting up my sushi guide which contains my recipe for sushi rice dressing, which I'll link here). And that's some 'Sunomono' you can see up top, which is basically just a really simple Japanese cucumber and wakame seaweed salad. You could even just serve it on it's own in half-portions, or with a tiny little sushi rice package as a sophisticated starter.
Right, what are you waiting for...
Seared, Marinated Tuna with Daikon Dressing and MizunaServes two, 224 calories per serving (although 34.5 calories per portion are in the marinade, which is discarded, so technically overstated and probably just under 200 calories. I hate counting calories...). Good quality salmon steaks would also work well with this recipe, although you need to re-calculate the calories if you're counting, at 184 per 100g.
Before you consider making this dish (whether you make it with tuna or salmon), I need to make you aware about the importance of choosing appropriate tuna (or salmon) when you're not cooking it all the way through to avoid food borne illnesses - please see underneath in the tips for more information. (Sorry, boring health and safety stuff! Y'know, in a 'do what I say, not what I do' kind of way... ;) )
- 2 good quality, sashimi grade fresh tuna (or salmon) steaks, about 2 ½ cm thick (or thicker, if you’re fond of sashimi!), weighing around 100-120g each [220-264 calories for both steaks, average 242]
- 1 tsp neutral oil 
- Small handful of mizuna leaves  (or substitute wild rocket, watercress, cress, shiso leaves if you can get them or just your favourite small salad leaves)
- 4 chives, snipped into short lengths 
- 1 spring onion, finely sliced on the long diagonal 
For the marinade
- 25ml sake (dry sherry if cooking gluten free, omit for Paleo) 
- 1 tbsp rice vinegar 
- 50ml light soy sauce (tamari if cooking gluten free/Paleo, or coconut aminos) 
- 1 small garlic clove, crushed 
For the daikon dressing
- 4 tbsp daikon – spiralized, julienned, or grated (depending on your preference. Have more if you're a big fan - I love it!) 
- 4 tsp lemon juice 
- 3 tbsp light soy sauce (tamari if cooking gluten free/Paleo, or coconut aminos) 
- 1tsp toasted sesame oil 
Mix together the marinade ingredients, and put the tuna steaks in it for 30 minutes to an hour,
Meanwhile, make the daikon dressing, by mixing the daikon and lemon juice together. Only add the soy sauce and sesame oil just before you serve it.
Remove the tuna from the marinade, and pat dry. Heat a heavy based (flat) griddle pan, or good non-stick pan over a medium heat, until VERY HOT (if you’re not sure, splash a few drops of water onto it, and see if they immediately hiss and sizzle). Coat the tuna steaks evenly with the oil, and then sear in the pan (you only want to cook the very outside, so that they are effectively raw in the middle) for 1-2 minutes on each side.
Slice each steak into slices about 1cm thick, arrange on a plate, top with a pile of daikon dressing in
the centre, and scatter the leaves, sliced spring onions and chives on top and around the tuna.
You could dip the tuna steaks into black, white, or a mixture of black and white sesame seeds before searing it - the oil will be sufficient to stick the majority to the tuna.
Regarding eating tuna or salmon which is not cooked all the way through, and in this case pretty much raw - technically speaking, you need to make sure you're buying sashimi grade fish (ask your fishmonger), because raw fish can cause food borne illnesses as it can contain bacteria and parasites. Sashimi grade means it has been frozen at a specific temperature, for a certain number of hours to kill parasites (which varies depending on European Union guidelines, the FDA etc.). I'm just telling you this so you can make an informed choice. You could choose to freeze it yourself for several hours, then defrost and use, or just take your chances if you're confident with the quality of the fish you buy.
You can buy all kinds of spiralizers, julienne peelers on t'internet. I do own a spiralizer, which makes pretty curly things out of vegetables. You could use a plain old grater on the coarse side, or use a mandolin or food processor on the fine julienne setting. It's going to end up tasting pretty much the same. I like to use my spiralizer for the daikon, as it reminds me of having tuna and salmon sashimi served with piles of spiralised daikon when eating out at a sushi bar!