Whole tuna steaks or salmon fillets can also be coated with the crust mixture.
Using a sharp knife, slice each tuna steak into two long pieces that are about 1 1/2 inches wide. Grind mustard seeds to a coarse powder in a spice grinder or mortar and pestle. Combine ground seeds with salt and pepper, and spread mixture out on a plate.
Dredge each piece of tuna in the ground mustard mixture, gently pressing the seeds against the tuna so that they adhere.
Place a large skillet over medium heat. Let stand until the skillet feels hot when you place your palm just above it, about 1 minute. Add 1 1/2 tablespoons olive oil to skillet. Heat oil until it is hot but not smoking, about 1 minute more.
Carefully arrange pieces of tuna in the skillet about 1 inch apart (it is easiest to cook 2 pieces at a time; if you do so, add 1 1/2 tablespoons oil for second batch). Sear tuna about 1 minute, until it releases easily from skillet and the seeds are deep golden brown. Continue process until the long sides are seared. (The tuna will be very rare in the center. If you want the tuna to be more well done, increase cooking time.)
Using a sharp knife, slice the tuna into squares. Arrange on plates with a handful of arugula and half a lemon. Divide remaining 1 1/2 tablespoons olive oil among four plates, drizzling it on arugula. Serve immediately.