Heat the oven to 375°F. Lightly grease a small baking dish or line it with parchment paper.
Finely chop the broccoli and cauliflower so the pieces are small enough to mix evenly into the meatloaf.
In a large bowl, whisk the egg, milk, Dijon mustard, garlic powder, salt, and black pepper. Stir in the breadcrumbs, chopped onion, broccoli, cauliflower, and shredded cheddar.
Add the ground turkey. Mix gently with your hands or a fork just until everything is combined. Avoid packing the mixture tightly, which can make the meatloaf dense.
Shape the mixture into a loaf in the prepared baking dish. Spread the ketchup over the top in an even layer.
Bake until the center reaches 165°F on an instant-read thermometer. Let the meatloaf rest for 10 minutes before slicing so it stays together.