Cook bacon to your preferred crispness, then transfer to a paper towel-lined plate to drain. Cut or tear each slice into halves, creating 8 strips.
Combine mayo, sour cream, mustard, and a pinch of salt and pepper in a small bowl. Whisk until smooth. Refrigerate until ready to use.
Lightly toast the bread in a toaster, then spread each side with butter. Set aside.
Spread about 1-1/2 tablespoons of sauce on each slice of toasted bread. Top one slice with lettuce. Add tomato slices and sprinkle with a tiny bit of salt. Add the bacon to your preference. Top with avocado slices and a touch of pepper. Place the remaining slice of bread on top and gently press down.
Use a sharp serrated knife to slice the sandwich in half. Enjoy immediately!
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Notes
Note 1: I love a good heirloom tomato best! If the slices are juicy, let them stand on a paper towel to slightly drain before adding them to the sandwich.Storage: This sandwich will not store well, it’s best to keep ingredients separate until you’re ready to eat.