Place butter and oil in an ovenproof casserole dish. Place in oven to melt butter.
Pound chicken to 1/2-inch thickness. Sprinkle with thyme, salt, and pepper. Place chicken in casserole and coat with butter mixture. Turn chicken skin-side up, if the chicken has skin, and sprinkle with lemon juice.
Bake 15 to 17 minutes or until chicken is cooked through. Transfer chicken to a serving platter and pour juices from dish over chicken. Garnish with lemon slices, if desired.