Trim crusts from bread. Butter each slice.
Grease a 7 x 11 inch pan. Place 6 slices of the bread, buttered-side down, in the pan.
Cook sausage and onion, breaking up sausage as it cooks. Drain.
Stir in pimientos.
Spread meat mixture evenly over bread slices.
Top with half of the cheese.
Place other 6 slices of bread, buttered side up, on top of the cheese layer.
In a large bowl, beat eggs, milk, and salt. Pour egg mixture over bread.
Top with remaining half of cheese. Cover and refrigerate up to 18 hours.
Bake in preheated oven at 325 degrees for 50 minutes. Serve Immediately.