If cooking over an open fire, allow the fire to reduce to embers. Place whole tomatoes and unsliced onion close to the hot embers and rotate until charred and blistered. If using a gas or electric grill, preheat grill to high heat. Slice tomatoes and onion, then drizzle with olive oil and grill until slightly charred.

Transfer charred tomato and onion slices to a blender. Add butter, garlic cloves, and chili flakes. Blend until smooth. Adjust seasoning with salt and pepper.