Tuna is probably one of the best fishes for blackening and by wonderful coincidence, no way of cooking tuna is better than blackening. The thing to know is that blackening fish (or anything else) creates a terrific amount of smoke and possibly flames. Best done outdoors over a very hot fire. - it can't possibly be too hot. A red-hot cast iron frying pan is ideal but you probably will not get it that hot.
4 tuna steaks, about 300g each, cut at least 2 cm thick (but better if thicker.)
100 ml white wine
3 dashes. Worcestershire sauce
2 tablespoons lemon juice
2 cloves garlic, finely chopped
1 tablespoon paprika (sweet or hot)
1 tablespoon black pepper
1 tablespoon salt
250 g melted butter
6 tablespoons softened butter
Cut off any dark parts of the tuna and discard.
Mix the wine, Worcestershire, lemon juice, and garlic in a broad bowl. Place the tuna steaks in this mixture for about thirty seconds on each side. Shake off excess marinade and reserve the tuna.
Strain the remaining marinade into a small saucepan and bring to a boil. Reduce by half. reserve.
Place a large cast iron frying pan over the hottest heat source you have. The pan is ready when the surface is smoking.
Combine the paprika and pepper with the salt in a bowl. Coat both sides of the fish liberally. Spoon melted butter over both sides, enough for it to drip a bit.
Place the fish into the hot skillet. BE CAREFUL! There is a very good chance that this will flame up briefly, most definitely there will be a lot of smoke. Don't touch the fish for a minute, or you will break the crust. After a minute or so it will break free when lifted with a spatula. Turn it and cook the other side the same way. Spoon a little more butter over - It should be pink in the center.
To make a lemon butter sauce, reduce the marinade by half, then remove from the heat. Mount the softened butter in this sauce a tablespoon at a time until it is creamy.