Combine salt, pepper, thyme, and rosemary with olive oil. Coat oxtails with herb mixture. Heat skillet with a lid or dutch oven to medium high. Add olive oil and sear oxtails on all sides.
Remove oxtails, set aside. Reduce heat to medium low, add onions and cook until translucent. Add minced garlic and cook 2 minutes, stirring constantly. Do not burn the garlic or it will taste bitter.
Add beef broth and bay leaf. Add oxtails. Bring to a simmer. Reduce heat to a low simmer, place lid on skillet, and cook for 2 1⁄2 hours.
Add chopped carrots. Place lid on skillet and cook 30 minutes. Mix cornstarch with a small amount of water or use the liquid in the skillet. Whisk into skillet to create a gravy. Add more cornstarch if needed. When gravy is thickened, serve.