Preheat the oven to 350°F and adjust the rack to the center position. Lightly grease a 13×9-inch glass baking dish. Arrange the biscuits in a single layer, pressing them together to form a unified crust that extends halfway up the sides of the dish. Place the pan on a baking sheet and bake for 12–15 minutes, checking frequently after the 12-minute mark, until the edges are just turning light golden. Remove from the oven and keep the oven on. While the crust bakes, cook the browned ground beef with taco seasoning and the specified water according to the packet instructions, then transfer the seasoned meat to a bowl and wipe out the skillet. In a separate bowl, blend the sour cream, mayonnaise, 4 ounces of shredded cheddar, and half of the sliced onion until thoroughly combined; set aside. Using the same skillet, sauté the remaining onion along with the sliced bell pepper until just tender. Assemble the casserole by layering the cooked taco meat over the baked biscuit crust, followed by sliced tomatoes, the sautéed peppers and onions, and the jalapeño slices. Spread the sour cream mixture over the top, then finish with the remaining shredded cheddar. Bake uncovered for 30–40 minutes, or until the edges of the crust are lightly browned and the cheese is fully melted and bubbling.