Heat the oven to 375 degrees and lightly grease a loaf pan. In a large bowl, combine the ground beef, grated zucchini, grated carrots, chopped onion, and diced red bell pepper.
Add the egg, breadcrumbs, milk, garlic powder, salt, and black pepper to the bowl. Mix with your hands until the ingredients are evenly distributed but not overworked.
Press the mixture into the prepared loaf pan and smooth the top. In a small bowl, stir together the brown sugar, ketchup, and Dijon mustard until smooth.
Spread the glaze evenly over the top of the meatloaf. Bake for 50 to 55 minutes until the internal temperature reaches 160 degrees.
Remove from the oven and let the meatloaf rest for 10 minutes before slicing.