Prep the oven and pan: Preheat to 400°F (200°C). Lightly grease a 9x13-inch baking dish.
Roast the veggies: Toss cauliflower, bell pepper, and red onion with oil, smoked paprika, a pinch of salt, and black pepper.
Spread on a sheet pan and roast for 12–15 minutes until crisp-tender. This step keeps the casserole from getting watery.
Make the keto BBQ sauce blend: In a bowl, whisk BBQ sauce, mayonnaise, cream cheese, apple cider vinegar, garlic powder, onion powder, and cayenne. Stir until smooth.
Taste and adjust salt and pepper.
Combine with chicken: In a large mixing bowl, add shredded chicken and the roasted veggies. Pour in the sauce and toss until everything is well coated.
Assemble the casserole: Spread the mixture into the baking dish. Sprinkle cheddar and mozzarella evenly on top.
Bake: Bake for 15–18 minutes, until the cheese is melted and bubbling around the edges.
Broil for color: If you like a golden top, switch to broil for 1–2 minutes.
Watch closely so it doesn’t burn.
Rest and garnish: Let it sit for 5 minutes to set. Garnish with chopped parsley or green onions.
Serve: Spoon into bowls and enjoy as is, or pair with a simple green salad or roasted broccoli.