In a medium bowl, whisk together the Dijon mustard, lemon juice, olive oil, minced garlic, thyme, salt, black pepper, and red pepper flakes until well combined.
Place the chicken breasts in a large resealable plastic bag or a shallow dish. Pour the marinade over the chicken, ensuring all pieces are well coated. Seal the bag or cover the dish with plastic wrap.
Refrigerate the marinating chicken for at least 1 hour, or up to 8 hours for more flavor.
Preheat the grill or oven to 375°F. If grilling, oil the grates to prevent sticking.
Remove the chicken from the marinade and discard the marinade. Grill the chicken for about 6-7 minutes per side, or bake in the oven for 25-30 minutes, until the internal temperature reaches 165°F.
Let the chicken rest for 5 minutes before slicing. Serve warm.