- Heat oven to 400 degrees.
- Slice grapefruit to loosen each segment. Whisk together butter, ⅓ cup syrup, cinnamon and walnuts.
- Place grapefruit halves on a baking sheet. Spoon topping onto grapefruit halves, distributing equally. Bake for 10 to 12 minutes, until very soft and lightly browned on top. Remove from baking dish and serve with additional syrup while warm.