For the guacamole: Add the avocado flesh to a bowl and smash. Mix in the shallot, tomato and lime juice. Season with salt and pepper and shake in some hot sauce.
For the nachos: Preheat the oven to 350 degrees F.
Spread the tortilla chips on a sheet pan and bake until hot, about 10 minutes.
Meanwhile, cook the sausage in a skillet, stirring, until browned, about 4 minutes. Remove from the heat.
Combine the cheese, scallions and cornstarch in a medium saucepan. Add 1 1/4 cups half-and-half and cook over medium heat, whisking constantly, until melted and smooth. Stir in the pimientos. Add more half-and-half if necessary for a nice drizzling consistency.
Transfer half of the chips to a large dish and top with half the sausage, beans, jalapeños and olives. Drizzle generously with the cheese sauce. Add the remaining chips, sausage, beans, jalapeños and olives. Drizzle on more cheese sauce, then top with the guacamole and sour cream. Serve additional cheese sauce on the side for dipping.