Prepare the chicken: Pat the chicken breasts dry and season with a pinch of salt, pepper, and garlic powder. Drizzle with olive oil and set aside while you heat the grill or a grill pan over medium-high heat.
Cook the bacon: In a large skillet over medium heat, cook the bacon until crisp. Transfer to a paper towel–lined plate to drain. Reserve a little bacon fat in the pan if you like a touch of smoky flavor when toasting the bread.
Grill the chicken: Grill the seasoned chicken breasts for 5–6 minutes per side, until they reach an internal temperature of 165°F (74°C) and have nice grill marks. Remove from heat and let rest for 5 minutes before slicing thinly.
Make the spread: In a small bowl, whisk together mayonnaise, Dijon mustard, lemon juice, and a pinch of garlic powder. Add salt and pepper to taste.
Toast the bread: Lightly butter or oil the bread slices if desired and toast on the grill or in a skillet until golden and crisp on both sides.
Prepare the avocado: Mash the avocado with a fork until creamy but still slightly chunky. Add a pinch of salt and a squeeze of lemon juice to prevent browning.
Assemble: Spread a thin layer of the mayo-mustard spread on two slices of bread. Layer lettuce, tomato, grilled chicken slices, bacon, and avocado. Top with the remaining bread slices and press gently to compact.
Slice and serve: Cut each sandwich in half on the diagonal for easy eating. Serve immediately with a light side like apple slices, pickles, or a simple salad.