One of the best preparations of rare tuna I ever had was Chile Crusted Tuna at Bobby Flay's Mesa Grill in New York 15 years ago.
I don't really have a recipe. It's my own inspired version of that great dish. It's really easy for the home cook. It works best on thicker cuts of tuna.
Rehydrate split three dried pasilla chilis (or something similarly sweet) and rehydrate in water.
In a food processor or spice grinder, grind one half a dried chipotle (seeds and veins removed), one half teaspoon of brown sugar, four cloves of garlic, whole toasted cumin, kosher salt, whole black pepper and some dried onion.
Chop the rehydrated pasilla chilis and add them to the food processor. Running it until all the chilis and dried ingredients have become a paste (you made need to add a little olive oil to achieve this depending upon how much liquid you have in your rehydrated chilis). You can make this wet rub up to a week in advance. I think that it develops better flavor as it sits for a few days.
Spread the mixture moderately thickly over one side of your tuna and let it marinate for 30 minutes to an hour.
In a hot saute pan, add a two count of olive oil per piece of fish and sear the tuna, chili side first for two minutes, then on the unseasoned side for two minutes. The tuna should be cold and rare in the middle, hot, spicy and crusty on the outside. Be really careful when you turn the tuna so as not to lost the pepper crust. I recommend using a spatula with a lot of surface area.
I like to serve it over a little garlicky sauteed spinach. and some of the now spicy searing oil from the pan.