Preheat oven to 375°F (190°C). In a large skillet over medium-high heat, cook the ground beef and chopped onion until the beef is no longer pink and the onion is soft, about 7-8 minutes. Drain any excess fat. Stir in garlic powder, salt, and pepper.

To the skillet with the beef, add the undiluted cream of mushroom soup, 1 cup of milk, and the thawed mixed vegetables. Stir until well combined and heated through, about 3-4 minutes. Remove from heat and stir in 1/2 cup of the shredded cheddar cheese until melted.

In a medium bowl, whisk together the flour, baking powder, and salt. Make a well in the center and pour in the 1/2 cup of milk and melted butter. Stir just until a soft, sticky dough forms. Do not overmix.

Pour the hot beef mixture into a greased 9x13 inch baking dish. Drop spoonfuls of the biscuit dough evenly over the top. It will not fully cover the surface. Sprinkle the remaining 1/2 cup of cheddar cheese over the dough. Bake for 25-30 minutes, or until the topping is golden brown and a toothpick inserted into the biscuit topping comes out clean.

Remove from the oven and let the casserole rest for 5-10 minutes. This allows the filling to set slightly, making it easier to cut into neat slices or squares for serving.