Flatten chicken breasts out using a meat tenderizer to about 1/4 inch thick. Place flour, salt, pepper, and garlic powder in a large ziplock bag. Shake to mix. Add chicken breasts and shake once more to coat chicken.
Melt butter in a large heavy-duty pan on medium-high heat. Add floured chicken breasts and cook for 3-4 minutes per side or until lightly browned. The butter will begin to foam then settle down. [br
Remove chicken from the pan and add the lemon slices and thyme to pan. Cook lemon slices for 1 about 1 minute per side. Return chicken to pan and top with the cooked lemons.