Preheat the oven to 375 degrees F. Lightly oil 12 muffin tins.
Add your onions, pepper, carrot, mushrooms and garlic to your food processor and pulse until all of the veggies are finely diced. (Alternatively, you can finely chop the veggies with a knife.)
Add the meat to a large bowl. Add in the veggies, egg, mustard, grain-free flour and spices (everything but the ketchup). Mix until well combined. We want it thoroughly mixed so the flavor is throughout the meat, but stirring the mixture too much will create a texture that it is more dense than you want.
Divide equally into the 12 greased muffin tins. Top each muffin with 1 tbsp of ketchup and spread gently to cover the top.
Cover with foil and bake for 30 minutes. Remove the foil and return to the oven for 5 more minutes. Use a meat thermometer to check to make sure the center is cooked to 165 degrees.
Remove from oven and let rest for 5-10 minutes. Then, use a spoon or a small rubber spatula to lift them out of the tins. Splash on some sriracha or hot sauce, if you like, when serving. Enjoy!