Chop the onion, carrots, and celery and mince the garlic; set all aside.
Heat 1 Tbsp butter in a deep skillet over medium heat and add the ground chicken. Season with salt, black pepper, red pepper flakes, and sage.
Allow the chicken to brown without stirring too often; after cooking through, remove the chicken with a slotted spoon, leaving the fat in the pan. Add 1 Tbsp butter if the pan is dry.
In the same skillet, sauté the diced onion until it becomes translucent. Add the carrots, celery, corn, garlic, and a pinch of salt and pepper; cook until the vegetables soften, around 5 minutes.
Break up the cooked chicken and return it to the skillet; mix everything well.
Add 2 Tbsp butter and the flour to the skillet; stir and cook for about 2 minutes until the mixture coats the veggies and the butter is melted.
Pour in the chicken broth, scraping the browned bits from the pan, and bring to a simmer until the mixture thickens slightly. Adjust seasoning as needed.
Transfer this filling into a casserole dish and set aside.
Preheat the oven to 425°F.
In a bowl, combine flour, baking powder, salt, pepper, and sugar, then toss in the shredded cheddar cheese.
Melt 2 Tbsp butter and mix it with the buttermilk; fold this wet mixture into the dry ingredients gently until just combined.
Spoon dollops of the biscuit batter evenly over the chicken mixture in the casserole.
Cut chilled butter into small pieces and scatter them over the biscuit topping.
Bake the casserole for about 25 minutes until the biscuits turn golden brown and cooked through.
