Fill a large stockpot two-thirds full of water. Add enough salt so that the water tastes like seawater. Add half of the sliced lemon and the reserved peels from the garlic and onion, and bring to a boil. Plunge the crabs in the water, in batches if necessary, and cook for 5 minutes. Remove the crabs from the stockpot and set aside until cool enough to handle.
Preheat the oven to 450 degrees F.
When the crabs are cool enough to handle, remove the top shell and apron and discard. Remove the gills and any transparent cartilage and scrape out any crab fat and rinse crabs under cool running water. Place crabs, bottom side down, on a cutting board and halve lengthwise with a heavy knife.
Combine 1 1/2 teaspoons of the salt, the minced garlic, onions, butter, olive oil, crushed red pepper, and black pepper in a large roasting pan. Place the pan in the oven on the lowest rack, and bake, stirring occasionally, until onions and garlic are golden, about 8 minutes. (Do not allow the garlic to burn.) Remove the pan from the oven and add the crab pieces and mix thoroughly so that the crabs are well coated with the garlic and butter mixture. Return the pan to the oven and bake, stirring occasionally, until the crabs are heated through and the garlic mixture is braised onto the shells, 13 to 15 minutes. Remove from the oven, sprinkle with the chopped parsley or cilantro, and serve hot.