1. Mash the avocado in a small bowl with the tomato, red onion, cilantro, lime juice, salt, pepper to taste.
2Season scallops with salt and black pepper. Add the butter and oil to a 12-inch skillet on high heat. Once the butter begins to smoke, gently add the scallops, making sure they are not touching each other. Sear the scallops for 1 1/2 minutes on each side. The scallops should have a 1/4-inch golden crust on each side while still being translucent in the center.
3.Spoon the guacamole onto the tostada, top with scallops and drizzle with sour cream.