Place the onions in a large ovenproof pan, cover halfway with water, and bake at 375° F for 1 ½ hours (90 minutes). Carefully remove the pan from the oven, and check the water level, adding more water as necessary to make it reach half way up the onions. Using tongs, turn the onions upside down, place back in the oven, and continue to roast for an additional 1 ½ hours (90 minutes), or until extremely tender. Remove the onions from the liquid (saving the onion broth for another use), and cool the onions completely. Once cool, remove and discard the outer layer of the onion. Using your finger, gently remove the inner rings of the onion, leaving two or three outer rings intact. Place the inner portion of the onion in blender and puree until smooth. Pass the puree through a fine-mesh sieve or strainer, and season to taste with salt and pepper.
Cut half of the green beans into quarters. Cut the asparagus into 1-2 inch pieces. Fold the quartered green beans, half of the asparagus, and half of the peas into the couscous. Fill each onion with some of the couscous mixture, spoon the olive oil along the outside of the onion, and bake at 350 ° F for 5 minutes, or until hot.
Place the onion puree in a small pan, and warm over medium heat.
Quickly sauté the remaining green beans, asparagus, and peas in 1 tablespoon butter over medium heat for 2 minutes, or until hot.
Assemble the Stuffed onions: Spoon some of the onion puree in the center of each plate. Place some of the hot vegetables in the center of the puree, and place the whole onion, mouth up, on top of the vegetables. Sprinkle sprouts or chopped fresh herbs over the top of the onions, and spoon some additional onion puree around the plate. Serve immediately.