If chicken breasts are uneven thickness, place each between plastic wrap and gently pound to an even ~1/2 inch thickness. Pat dry with paper towel. Rub both sides with 1 tablespoon olive oil, 1 tsp salt, 1/2 tsp black pepper, 1 tsp garlic powder, and 1 tsp chopped fresh rosemary.

Cook the chicken

Heat a large skillet over medium-high heat. Add remaining 1 tablespoon olive oil. When shimmering, add the chicken breasts and cook 4–5 minutes per side until golden and internal temperature reaches 165°F (74°C). Remove to a cutting board and let rest 5 minutes, then slice thinly against the grain into sandwich-sized pieces.

Cook bacon

In a separate skillet or on a baking sheet in a 400°F (200°C) oven, cook bacon until crisp (about 8–12 minutes in oven or 6–8 minutes in skillet over medium-high). Transfer cooked bacon to paper towels to drain.

Prepare mayo & vegetables

Make rosemary mayo: in a small bowl mix 1 cup mayonnaise with remaining 1 tsp chopped rosemary and a pinch (about 1/4 tsp) black pepper. Taste and adjust seasoning. Slice tomatoes into 1/4-inch thick slices. Rinse and dry lettuce leaves.

Toast and assemble

Optional - spread softened butter lightly on one side of each bread slice. Toast the 8 bread slices in a skillet or toaster until golden and slightly crisp, buttered side down if using skillet, about 2–3 minutes per side.

To assemble each sandwich: spread about 2 tablespoons of rosemary mayo on one slice of bread. Layer 1–2 leaves of lettuce, 2–3 tomato slices (season tomatoes lightly with a pinch of salt and pepper, if desired), 1/2 of the sliced rosemary chicken (approximately half a breast worth), and 2 slices of crisp bacon. Spread another ~1 tablespoon mayo on the top slice of bread and close the sandwich.

Press lightly, cut sandwiches in half on the diagonal, and serve immediately.