Preheat the oven to 400°F. Put the cauliflower florets in a single layer in a large roasting pan and drizzle with 3 Tbsp. of the olive oil. Bake, stirring occasionally, until browned and tender, 25-30 minutes. Remove from the oven and stir in the butter, garlic, and red pepper flakes. Return to the oven and roast until the garlic is fragrant, about 5 minutes. Stir in the kidney beans, parsley, and chives. Season with salt and pepper. Set aside.

In a medium-size heavy skillet, heat the remaining 1 Tbsp. olive oil over high heat. Add the radicchio and sauté until evenly browned, 2-3 minutes per side. Add the vinegar and cook until reduced and thick, about 5 minutes, turning the radicchio several times.

Place a radicchio wedge on each of 6 small plates. Spoon an equal amount of the cauliflower mixture over or next to each wedge and serve.