Under cold running water, wash and scrub 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 the water, cover, and cook until color changes to red. Drain well and allow to cool to touch.
Carefully remove the upper top shells. Under cold running water, clean well and pat dry. Set aside.
Pick through the shells, cartilage, and claws to extract the crab meat. Set aside.
In a pan over medium heat, heat butter until it begins to melt. Add onions, garlic, and bell peppers and cook until softened. Remove from heat and allow to cool completely.
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.
Notes
Breadcrumbs act as a binder and extender. If the amount of crab meat is not enough to fill up the shells abundantly and you prefer not to add too much breadcrumbs, you can supplement it with canned or fresh lump crabmeat from your supermarket.