Place each chicken breast between two pieces of plastic wrap and flatten with a meat tenderizer or rolling pin to about 1/4-inch thickness.
Combine the flour, salt, black pepper, and garlic powder in a large zip-top bag and shake to mix.
Add the flattened chicken breasts to the bag, seal, and shake once to coat each piece evenly with the flour mixture; shake off excess flour.
Melt the butter in a large heavy skillet over medium-high heat until foaming subsides.
Add the floured chicken to the hot skillet and cook 3–4 minutes per side, until lightly browned and cooked through.
Remove the chicken from the skillet and set aside. Add the lemon slices and optional thyme to the pan and cook about 1 minute per side, until lightly caramelized.
Return the chicken to the skillet and spoon the cooked lemon slices and pan butter over the top to coat, then remove from heat and serve.
Serve with rice, vegetables, or a salad if desired.
