Preheat the oven to 375°F. Slice the tops off the garlic so the cloves are exposed. Place the heads in a baking dish and drizzle with olive oil. Cover the pan with foil and roast until the garlic is golden brown and fragrant, 20 to 25 minutes. Remove the garlic from the oven. Cool briefly before squeezing the cloves out of their skins into the bowl of a blender. Combine the vinegar and mustard, blending until smooth. With the machine running, slowly drizzle in 3/4 cup olive oil. Keep the blender running and season with salt and pepper.
