Step 1: Prepare the Spread
In a small bowl, mix mayonnaise, Dijon mustard, and lemon juice. Season with a pinch of salt and pepper. Set aside.
Step 2: Assemble the Sandwich
Lay out the bread slices and spread a thin layer of the prepared spread on one side of each slice.
Place a slice of cheese on two of the bread slices. Top each with chicken, avocado slices, tomato, spinach (or lettuce), and crispy bacon if using.
Add the remaining cheese slice on top and cover with the other bread slices, spread side down.
Step 3: Toast the Sandwich
Heat a skillet or griddle over medium heat. Lightly butter the outside of each sandwich.
Place the sandwiches in the skillet and cook for 3-4 minutes per side, pressing gently with a spatula, until the bread is golden brown and crispy, and the cheese is melted.
Step 4: Serve
Slice the sandwiches in half and serve immediately with your favorite side, such as chips, fries, or a salad.