Heat the oven broiler. Roast the poblano peppers, 4-5 minutes per side, until skin is all blistered and peppers are soft. Transfer to a ziploc bag and let them steam. Once cool enough to handle, peel and remove seeds. Blend the peppers with the sour cream, buttermilk, and a pinch of salt and pepper. Set aside.
Once the Guacamole is prepared, stir in the red onion. Cover with plastic wrap, directly on the guac to prevent browning, and set aside.
In a bowl, season the shrimp well with Old Bay, salt, pepper, chili powder, and smoked paprika (~1/4 tsp each with a bit more Old Bay and chili powder). Heat a large, nonstick skillet on high heat. Add the olive oil and shrimp and cook, tossing occasionally, until shrimp are just cooked through (~3 minutes). Remove from the heat.
To assemble the chalupa, spread a layer of refried beans on a chalupa shell, then a layer of Guacamole. Top with red cabbage, 5-6 shrimp, a drizzle of Roast Poblano Sauce, and a sprinkling of green onion. Serve immediately. Yields 4 servings.