Under cold running water, wash crabs to remove grit and debris. Drain well.
In a large pot over medium heat, bring about 1 quart of salted water to a boil. Lower crabs into water, cover and cook until color changes to red. Drain well and allow to cool to touch. Carefully remove upper top shells. Under cold running water, clean well and pat dry. Set aside.
Pick through the shells, cartillage and claws to extract the crab meat. Set aside.
In a pan over medium heat, heat butter until it begins to foam. Add onions, garlic and bell peppers. Cook until softened. Remove from heat and allow to completely cool.
In a bowl, combine crab meat, onion/bell pepper mixture, bread crumbs, mayonnaise and green onions. Add lemon juice and season with salt and pepper to taste. Stir to combine.
Divide mixture and stuff into each crab shells. Arrange in a single layer on a baking sheet and bake in a 375 F oven for about 15 to 20 minutes or until heated through and tops are golden. Serve hot.