Preheat oven to 400° Fahrenheit. Lightly oil a rimmed baking sheet and set aside.
In a medium bowl, combine the Brussels sprouts, garlic, and oil until well combined. Place the Brussels sprouts on the prepared baking sheet in a single layer, cut side down. Season with salt and pepper to taste. Bake in the oven, stirring occasionally,
until crispy and golden brown, about 20 minutes. Adjust seasonings with salt and pepper to taste.
While the Brussels sprouts are roasting, make the dipping sauce: In a small bowl, stir together the sour cream, mayonnaise, horseradish, sriracha sauce, garlic powder, and cayenne pepper, until well combined. Season with salt and pepper to taste.
Serve the Brussels sprouts at once with the creamy sriracha dipping sauce on the side.