Preheat your oven to 350°F (175°C) and grease a baking dish with butter or non-stick spray.
In a skillet over medium heat, melt 1 1/2 teaspoons of butter. Add chopped onion and sauté for about 5 minutes, until softened.
In a large mixing bowl, combine the sautéed onion, cooked chicken, cream of chicken soup, mayonnaise, milk, and chopped roasted red peppers. Stir to coat the chicken evenly.
Spread the chicken mixture evenly in the greased baking dish. Bake in the oven for 15 minutes.
Remove the dish from the oven. Sprinkle 1 cup of shredded cheddar cheese over the chicken mixture. Arrange the biscuit dough on top and sprinkle with another 1/2 cup of cheddar cheese.
Return the dish to the oven and bake for 20 minutes, or until biscuits are golden brown and cooked through, the cheese is bubbly and the filling reaches 165°F.
Let the casserole sit for a few minutes before serving. Serve hot and enjoy with a side salad if desired.
Notes
This casserole is great for leftovers. Refrigerate within 2 hours and eat within 3 to 4 days; reheat to 165°F before serving.