Preheat the oven to 375°F (190°C).
In a mixing bowl, combine the cream of chicken soup, lemon juice, capers, 1/2 cup of the grated Parmesan, garlic powder, salt, and pepper until smooth.
Lay the chicken breasts flat in a large casserole dish in a single layer.
Pour the soup-and-lemon mixture evenly over the chicken, spreading gently to coat.
Sprinkle the remaining 1/4 cup Parmesan over the top, then place two thin lemon slices on each breast if using.
Bake uncovered for 35–45 minutes, until the chicken reaches 160°F (71°C) internally and the edges are bubbly.
Remove from the oven, discard the lemon slices if desired, sprinkle with chopped parsley, and serve.