Step 1Mash the avocado flesh with the tines of a fork and add the lemon juice. Add the onion and chives. Add the oil in a stream, stirring to blend. Add the salt and pepper to taste.
Step 1Place the peppercorns, their juice, the mustard and the lemon juice in a blender. Add salt and pepper to taste. Blend at low speed for 30 seconds. Add the oil in a slow stream, blending on high speed. Taste and adjust the salt and pepper.
Step 1Slice the radishes as thinly as possible, using a mandoline if you have one.
Step 2Make a ring with the radishes in the center of each of 4 chilled plates. Place a 2 1/2 inch-diameter ring mold in the center of each ring of radishes. Divide the avocado relish among the molds; it should not be more than one-half-inch tall.
Step 3Season the tuna with salt and pepper. Add the green peppercorn dressing to taste, about 1 1/2 tablespoons. Add the chives and onion. Mix well. Adjust the seasoning. Divide the tuna among the 4 molds and smooth the tops. Remove the molds. Dress the greens very lightly with about 1 tablespoon of the dressing. Place the greens decoratively over the tuna. Drizzle the remaining dressing around the plates. Serve immediately.