Step 1In a small bowl, whisk together the mustard, soy sauce, ginger, shallots and 1 tablespoon of the lime juice. Continue whisking and slowly drizzle in one-fourth cup of the oil. (This makes one-half cup dressing.) Set aside.
Step 2In a medium bowl, toss the mesclun and sprouts with the remaining 1 tablespoon lime juice and 1 tablespoon oil. Add freshly ground black pepper to taste.
Step 3Lightly dredge the tuna in the cracked Sichuan pepper and a little salt on all sides of the tuna. Heat a pan over high heat until hot and lightly coat it with oil. Sear the tuna on each side for 30 seconds.
Step 4Slice the tuna into half-inch slices with a serrated knife and arrange on a plate. Garnish with the salad, soy-mustard sauce and the soybeans, if you like.