To make the lemon grass sauce, heat 1 tablespoon of the peanut oil in a small saucepan on medium-high heat. Saute the green onions, lemon grass, red bell pepper, ginger, and garlic until soft, about 2 minutes. Add the chicken stock, rice wine vinegar, soy sauce, sugar, chili garlic paste, and black beans. Bring to a simmer for 5 minutes. Add the dissolved cornstarch and simmer, stirring frequently, until the sauce becomes glossy and thickens, about 2 more minutes. Set aside.
To wok-fry the crab, heat 2 tablespoons of peanut oil in a wok over high heat until almost smoking. Toss in the crab pieces (including the carapace or top shell), add the sake, and cover. Allow to steam 2 to 3 minutes. Add the lemon grass sauce and simmer until the crab is heated through and the sauce thickens to desired consistency, about 1 to 2 minutes.
On a large plate or platter, arrange the crab with the carapace (top shell) back on top so it looks roughly like a whole crab again and pour all the sauce over and around it. You'll need crab-crackers, a side bowl for the shells, a stack of napkins, and bowls of warm lemon water to clean your hands. Serve this with steamed white rice.