Tuna with Mustard Seed Crust

1. 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.
2. Dredge each piece of tuna in the ground mustard mixture, gently pressing the seeds against the tuna so that they adhere.
3. 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.
4. Carefully arrange pieces of tuna in the skillet about 1 inch apart. 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.
5. 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.

Serves 4.