PREP TIME: 0:30 MIN
- Trim tuna, removing skin, gristle, and any dark meat.
- Combine mustard, thyme, ground fennel, cayenne, and salt in a small mixing bowl. Stir well to combine.
- Heat a large skillet over moderately high heat. Coat bottom of skillet with a thin film of olive oil. When oil is hot, cook tuna fillet about 2 minutes per side, until nice and golden on the outside and rare to medium-rare on the inside.
- Slice cooked tuna filet into 1/4 inch slices.
- Serve on toasted sesame rolls or English muffins, slathered with mustard aioli.