Step 1: Toast the Bread
Lightly butter one side of each bread slice. Toast in a toaster, toaster oven, or skillet over medium heat until golden brown and crispy, about 2-3 minutes. Set aside to cool slightly while you prep the other ingredients.

Step 2: Prepare the Vegetables
Wash and thoroughly dry your lettuce leaves—this is crucial for preventing soggy sandwiches. Slice your tomato into 1/4-inch thick rounds and place them on paper towels to absorb excess moisture. Slice your cucumber and red onion as thinly as possible for the best texture.

Step 3: Prepare the Avocado Spread
In a small bowl, mash the avocado with a fork until mostly smooth but still slightly chunky. Season with a pinch of salt and black pepper. You can add a squeeze of lime juice to prevent browning and add extra flavor.

Step 4: Assemble the First Layer
Place one slice of toasted bread on your work surface, toasted side up. Spread with 1 tablespoon mayonnaise. Layer with 1 slice of cheese, 2 lettuce leaves, 2-3 tomato slices, and several cucumber slices. Season lightly with salt and pepper.

Step 5: Add the Middle Layer
Place the second slice of bread on top of the first layer. Spread the top with half of the mashed avocado. Add another slice of cheese, remaining lettuce, tomatoes, cucumbers, and red onion slices. If using sprouts or bacon bits, add them here.

Step 6: Create the Top Layer
Spread mustard on one side of the third slice of bread and place it mustard-side down on top of the sandwich. Gently press down to compress the layers slightly so they stick together.

Step 7: Secure and Cut
Insert 4 toothpicks or sandwich picks through the sandwich, positioning one in each quarter. Using a sharp serrated knife, cut diagonally from corner to corner to create 4 triangular sections. Each toothpick should hold a quarter together.

Step 8: Serve Immediately
Arrange the quarters on a plate with the colorful layers facing out. Serve with potato chips, pickles, or a side salad. Enjoy immediately while the bread is still crispy!