In a large mixing bowl, combine the ground beef, breadcrumbs, milk, egg, chopped onion, minced garlic, salt, pepper, and dried thyme (if using). Add the bell pepper if desired.
Mix all ingredients together until evenly combined.
Form the mixture into a loaf shape and place it into a greased baking dish or on a lined baking sheet.
Spread a thin layer of ketchup over the top of the meatloaf.
Bake in the preheated oven for about 45-60 minutes, or until the meatloaf is cooked through (an internal temperature of 160°F/71°C).
Let it rest for a few minutes before slicing and serving. Enjoy!