Prep your tools. Spray a 9×13 baking dish with oil. Preheat the oven to 325°F.
Saute the veggies. Heat the oil in a large skillet, add chopped onion and chopped bell pepper and saute for about 5 minutes to soften.
Add ground beef to the skillet. On medium-high heat cook the beef until the ground beef is fully cooked. Add 3/4 water and taco seasoning to the skillet, along with the RoTel tomatoes. Bring to a boil, then lower the heat and simmer until it thickens. This will take about 10 minutes.
Make the biscuit layer. Combine Bisquick mix with water and make the dough. Spread the dough over the baking dish and press to the bottom.
Spread the ground beef mixture over the dough layer. Add the layer of sliced tomatoes and green chili peppers.
Make the sour cream layer. In a separate bowl mix mayonnaise with sour cream and a half mozzarella cheese. Spread that mixture over the tomato/chili layer.
Top with cheese. Sprinkle with cheddar and remaining mozzarella cheese.
Bake. Bake the casserole for 35-40 minutes in the preheated oven.
