Preheat the oven to 325°F. Lightly grease a 9x13-inch baking dish.
Pat the chicken breasts dry with paper towels, then season all sides with salt and pepper. Arrange the breasts in a single layer in the prepared baking dish.
Bake the chicken uncovered for 20-25 minutes.
Meanwhile, heat the coconut or vegetable oil in a medium saucepan over medium heat. Add the onion and garlic. Cook, stirring frequently, until the onion is softened and translucent, about 5-7 minutes.
Stir in the chili sauce, honey, soy sauce, mustard, horseradish, and crushed red pepper flakes. Season to taste with salt and pepper.
Bring the mixture to a simmer. Cook, stirring occasionally, until slightly thickened and glossy, 5-10 minutes. Remove from heat and taste, adjusting seasoning with salt and pepper as needed.
Transfer about half of the sauce to a clean bowl, cover, and set aside for serving. Keep the remaining sauce in the pan for brushing the chicken.
Remove the chicken from the oven and brush all over with the sauce from the pan, coating each piece generously.
Return the chicken to the oven and bake for another 20-30 minutes, until the barbecue sauce is dark and sticky, and an instant‑read thermometer inserted into the thickest part of the breasts (without touching bone) registers at least 165°F.
Transfer the chicken to a platter and let rest for 10 minutes.
Serve warm with the reserved barbecue sauce on the side.