Combine mayonnaise, sour cream and melted bacon grease, onion, and chopped bacon and stir well.
Slice bread into 25-30 rounds per loaf. Spread mayonnaise onto bread rounds, top with a tomato slice, sprinkle with salt and pepper.
These can be made one day ahead. Arrange sandwiches on a cookie sheet in a single layer and cover well with plastic wrap. If holding, wait to season sandwiches with salt and pepper until just before serving.
Sprinkle lightly with parsley or basil just before serving.