For the mustard miso:
- ¼ cup white miso
- 2 tablespoons grainy mustard
For the pastrami rub:
- ½ cup light brown sugar
- 2 tablespoons smoked paprika
- 2 tablespoons brown mustard seeds, lightly cracked (in a mortar and pestle)
- 2 teaspoons kosher salt
- 1½ teaspoons freshly ground black pepper
For the tuna:
- 6 sushi-grade tuna fillets, about 6 to 8 ounces each
- 2 tablespoons vegetable oil
For the mustard miso: Combine the miso and mustard in a small bowl and set aside.
For the pastrami rub: Mix the rub ingredients in a small bowl and set aside.
For the tuna: Pat the tuna fillets dry with paper towels. Rub with the miso mixture evenly over all sides of the fillets. Dust the fillets with the pastrami rub to generously coat them on all sides.
Heat the oil in a large nonstick skillet over medium-high heat. Add the fillets and sear until browned on both sides but still rare in the middle, about 1 to 2 minutes on each side.
Make ahead: The mustard miso can be made the day before and stored in a sealed container in the refrigerator. The pastrami rub can be combined 1 to 2 days in advance and stored in an airtight container at room temperature.