Step 1

Arrange a rack in the middle of the oven and heat to 400°F. Place the onion in a large ramekin or small baking dish and drizzle with oil. Roast until the onion is dark on the outside and soft on the inside, 40 to 45 minutes. Cool completely.

Step 2

Peel and trim the onion. Place in the bowl of a food processor fitted with the blade attachment or a blender. Process until the onion is puréed with a smooth texture, 30 to 45 seconds. Add the maple syrup, mustard, salt, and pepper. Process until the sauce is well-combined, about 30 seconds. Serve immediately or store in a covered container in the refrigerator until ready to serve.