Make sure your oven is at 375 degrees Fahrenheit. Prepare a baking dish that is 13 inches by 9 inches with nonstick spray.

Cook the ground beef in a large skillet, breaking it up as it cooks. When the ground beef is almost done cooking, add the onion and continue cooking until he onion is tender and the ground beef is cooked through. Salt and pepper to taste after draining, if necessary.

Put 3/4 cup of water, the tomato paste, and the tomato sauce into a pot. For 15 minutes, stirring occasionally, simmer over low heat.

In the bottom of the prepared baking dish, spread half of the hot beef mixture. Spread the vegetables out evenly, then top with 2/3 cup of the mozzarella. Add another 2/3 cup mozzarella on the top. Cover it with the rest of the meat mixture. The remaining cheese should be sprinkled on top.

Break open the biscuits and cut them in half lengthwise so you have 16 smaller biscuits. Place the biscuits on top, overlapping one another, in three rows of five. (You'll end up with half a biscuit.)

Spread the melted butter on the biscuits with a pastry brush. Add some oregano for flavor. About 25 minutes in the oven should do the trick for a golden brown casserole.