Whisk milk and ancho chile and garlic powders in a shallow bowl until combined; add crabs. Cover and chill 1 hour.
Meanwhile, brush red onion slices with 1 tablespoon oil. Grill or broil 4 minutes per side. Set aside. Spread cut sides of rolls with butter, and grill or toast until golden brown.
Combine cornmeal and next 3 ingredients in a food processor. Process until mixture resembles fine crumbs; transfer mixture to a shallow dish. Drain crabs, and discard milk mixture. Dredge crabs in cornmeal mixture.
Heat 2 tablespoons oil in a large nonstick skillet over medium-high heat. When oil is very hot, add half the crabs, top-side down; sauté 2 to 3 minutes per side or until golden. Drain crabs on paper towels, then transfer to a baking sheet, and keep warm in a 200° oven. Repeat with remaining oil and crabs.
Spread toasted sides of rolls with Chipotle Tartar Sauce. Top with a lettuce leaf, 2 slices each tomato and avocado, a grilled red onion slice, and a fried crab. Top with sprouts and a squeeze of lime.
Wine note: "When I hear that a seafood dish has a kick, I reflexively go to the fire-extinguishing quality of a German Riesling. A light, off-dry Riesling such as Dr. Loosen Riesling Kabinett ($20) or Gunderloch Jean-Baptiste Riesling Kabinett ($16) will tame the heat emanating from the chile and garlic powders in this preparation. These wines' mouthwatering tanginess--so admirable in a good Riesling--will refresh the palate after a mouthful of fried crab." --Mark Oldman