Mix the fennel seeds, mustard seeds, and peppercorns in a mortar.
1 tsp. fennel seeds, 1 tsp. mustard seeds, ¼ tsp. black peppercorns
Coarsely grind everything, add the sesame seeds, and season to taste with sea salt.
4 tbsp. sesame seeds, Sea salt
Spread the spice mixture evenly over the whole surface of a plate and press the tuna steaks into the mixture to coat.
4 tuna steaks
Heat the coconut oil in a skillet placed over medium-high heat.
3 tbsp. coconut oil
Brown each tuna steak for 2 to 3 minutes per side or until they reach the desired doneness.