Preheat your oven to 375°F (190°C). Generously spray a 12-cup muffin tin with non-stick cooking spray. This step is crucial—nobody wants to chisel meatloaf out of their muffin tin.
Mix the meatloaf base. In a large bowl, combine ground beef, ground pork (if using), diced onion, garlic, bell pepper, eggs, breadcrumbs, ketchup, Worcestershire sauce, Dijon mustard, thyme, salt, and black pepper. If using any optional mix-ins like cheese or bacon, add them now.
Don't overwork the meat. Using your hands, gently mix until just combined. Overworking will result in tough meatloaf muffins.
Fill the muffin cups. Divide the meat mixture evenly among the 12 muffin cups. A cookie scoop or ice cream scoop makes this job easier and neater. Fill each cup to the top, creating a slight dome.
Create an indentation. Use your thumb or the back of a spoon to make a small depression in the top of each meatloaf muffin. This will hold the glaze and prevent it from sliding off.
Make the glaze. In a small bowl, whisk together the ketchup, brown sugar, apple cider vinegar, and garlic powder until the sugar dissolves.
Add the glaze. Spoon about 1 teaspoon of the glaze onto each meatloaf muffin, spreading it to cover the top.
Bake to perfection. Place the muffin tin in the preheated oven and bake for 20-25 minutes, until the internal temperature reaches 160°F (71°C) or the meat is no longer pink in the center.
Let them rest. Remove from the oven and let the meatloaf muffins rest in the tin for 5 minutes. This helps them set up and makes removal easier.
Gently remove from tin. Use a butter knife to loosen the edges if needed, then carefully lift each muffin out. Serve hot, with additional glaze on the side if desired.