For the Sauce: Heat olive oil in a medium saucepan over medium heat. Add onions and garlic and cook until soft. Add pasilla and ancho chile powders and cook for 2 minutes. Add wine and cook until almost dry. Add the cream and bring to a simmer. Add the mustard and cook for 5 minutes. Season with salt and pepper to taste.
For the Crabs: Place the flour, cornmeal and eggs in separate bowls and season each with salt and pepper to taste. Dredge each crab in flour, shake off any excess and dip into beaten eggs, covering completely. Dredge in cornmeal. Heat 2 tablespoons of the oil and 1 tablespoon of the butter in a large saute pan over medium high heat until it begins to smoke. Saute 4 crabs at a time until crisp, 2 to 3 minutes on each side. Remove to a platter lined with paper towels. Wipe out the pan each time with a clean cloth or paper towel. Place watercress in a bowl and toss with vinegar and oil and season with salt and pepper to taste. Arrange dressed watercress on a large platter, place crabs on top. Serve with Red Chile-Mustard Sauce.