• Preheat oven to 325° Fahrenheit. Light grease 9x13 baking dish with neutral-tasting cooking spray. Set dish aside.

For the Taco Meat

• Heat large skillet on stovetop over medium-high heat. When pan is warm, add 2 pounds lean ground beef. Cook, stirring and breaking up ground beef into small pieces, until meat is no longer pink, approximately 8 to 10 minutes.
• When ground beef is browned, drain grease from skillet then return pan to heat. Add 1 1-ounce packet taco seasoning, ¾ cup water, and additional salt only if needed.
• Stir to incorporate ingredients, making sure ground beef is well coated with seasoning, then let mixture simmer 3 to 4 minutes or until water has reduced. Once reduced, move skillet to cool burner and set aside.

For the Sautéed Veggies

• Place medium skillet on stovetop over medium-high heat. When pan is warm, add 1 tablespoon neutral-flavored oil and swirl or tilt pan as needed to distribute oil across pan surface. Continue heating pan until oil is hot and shimmery.
• When oil is hot, add 1 cup thinly sliced white onion and 1 cup red bell pepper slices to skillet. Stir to coat onions and peppers in oil, then sauté peppers and onions approximately 10 minutes or until soft and browned. Stir peppers and onions every 2 minutes or so to prevent sticking.
• Once onions and peppers are ready, remove skillet from heat and set aside.

For the Sour Cream Layer

• Add ½ cup sour cream , ½ cup mayonnaise, 4 ounces shredded cheddar cheese, and 1 cup thinly sliced white onion to medium mixing bowl. Stir all ingredients together until well incorporated. Set bowl aside.

For the Biscuit Layer

• Add 2 cups Bisquick buttermilk biscuit mix, 1 pinch salt, 2 ounces shredded cheddar cheese, and 1 cup water to large mixing bowl. Mix together ingredients until fully combined into soft dough.
• Transfer dough to prepared baking dish. Gently press dough out to cover entire bottom of dish in one even layer, pressing dough up each side of baking dish by ½ inch.

To Assemble and Bake the Casserole

• Transfer taco meat to baking dish, spreading taco meat out into one even layer covering biscuit layer completely.
• Arrange 2 medium tomatoes (diced) evenly across top of taco meat layer, then cover tomato layer with sautéed peppers and onions. Sprinkle 1 4-ounce can diced green chilies across peppers and onions layer.
• Top dish with sour cream mixture and spread mixture out evenly across top of casserole, then sprinkle remaining 2 ounces shredded cheddar cheese across top of sour cream mixture.
• Place baking dish in preheated oven. Bake casserole 30 minutes, or until slightly browned around edges.
• When casserole is browned around edges, carefully remove baking dish from oven. Let stand 3 to 5 minutes, then divide casserole into preferred portion sizes and serve warm.