Whisk together the olive oil, lemon juice, garlic, brown sugar, dried thyme, and red pepper flakes, if using. Set aside.
Pat the chicken breasts dry with a paper towel and season with the salt and pepper.
Heat the butter in a large skillet over medium-high heat until melted and hot.
Add the chicken to the skillet and sear on the first side for about 3-4 minutes, until golden brown.
Flip the chicken over and brush on the garlic marinade. Drizzle the extra around the chicken in the pan.
Lower the heat to medium and continue cooking the chicken, until no longer pink in the middle and the internal temperature reaches 165°F, about 5-10 minutes depending on the thickness of your chicken.
Serve hot, drizzling any excess pan juices over the chicken, if desired.