First thing to do is "dismantle" the crabs before you wok-fry them. Toss the live crabs into salted boiling water for 2 minutes. Next, place the crabs top-side up, and stick your thumb under the edge of the top shell, pull forward and lift the shell up and off, and reserve. Scrape out the gills that are found on top of the body. Now, turn the crabs over and on the underbelly you will find the "apron," a slightly-lifted triangular flap, pull this off too. Finally, rinse the crabs of all the grey or green spongy stuff (the soft yellow matter is fat or crab butter and considered desirable by many, keep it if you wish.) Divide the crabs into quarters with a big knife, leaving the legs attached to the 4 sections.
Heat the peanut oil in a wok over high heat until almost smoking. Add the garlic, ginger, chiles, and bok choy. Stir-fry for 1 minute, then remove to a side platter. Toss in the crab pieces, including the top shells. Take the top shells out after 1 minute, continue to stir-fry the crab for 3 minutes. With a big spoon or spatula, remove the crab pieces to the side platter. Now on to the sauce, pour 1 cup of water into the wok, along with the brown sugar, soy sauce, lemon juice, and sake. Stir for 2 minutes to dissolve the sugar. Stir in the cornstarch slurry and cook another minute until the sauce thickens. Return the crabs and bok choy to the pan, toss everything together to coat. Cover, and cook for 3 minutes.
To serve, line a large platter with butter lettuce leaves, and arrange the crabs with the top shell back on top so it looks roughly like a whole crab again - cool right?! Put the bok choy around the crab and pour the sauce all over the top. Garnish with the green onions and peanuts. You'll need crab crackers, mini forks, a side bowl for the shells, a stack of napkins, and bowls of warm lemon water to clean your hands. Serve with steamed white rice.