Heat a griddle over medium-high heat for 5 minutes. Crush peppercorns with the back of a heavy skillet or grind coarsely with a peppermill. Mix pepper with salt and pat all over tuna loin to form a crust on all sides. Sear on all sides, about 5 minutes in all. Remove and cool completely. Tuna should be nicely seared on the outside and rare on the inside. In work bowl of a food processor combine green peppercorns, vinegar and mustard. Process to a smooth paste. With motor still running, add oil in a slow, steady stream and process just until oil is absorbed and vinaigrette thickens. Taste and season. Toss greens with 2 tablespoons of dressing and mound on a chilled dinner plate. Slice tuna into 1/2-inch thick slices and top salad. Drizzle with remaining dressing and sprinkle with croutons.