Place the garlic and the olive oil in a small oven proof dish and put in the oven to slowly cook until the garlic is soft and fragrant, about 1 hour
Meanwhile, char the bell peppers over the burner or under the broiler until the skin is blistered, transfer to a container with a lid (or plastic wrap) to steam for 5 minutes, then scrap away the skin and seeds reserving as much of the liquid as possible
Combine the peppers, cooked garlic, olive oil, mustard and vinegar in a blender with a hefty pinch of salt and blend until smooth and emulsified