Place the grapefruit halves cut-side up on the prepared baking sheet, nestling them close together so they stay upright.
In a small bowl, stir together the brown sugar substitute, ground cinnamon, and sea salt until well combined and no clumps remain.
Add the melted butter and vanilla extract (if using) to the bowl and mix until a thick, spreadable caramelized mixture forms. It should look like a loose paste rather than a liquid; if it seems too runny, add another teaspoon of brown sugar substitute.
Using a spoon or small spatula, spread this one caramelized mixture evenly over the cut side of each grapefruit half, covering the surface all the way to the edges. Press it lightly so it adheres to the fruit—this is the part that will bubble and brown under the broiler.
Place the baking sheet under the broiler and cook for 4 to 7 minutes, watching closely. The topping should melt, bubble, and darken in spots, turning glossy and caramelized. Rotate the pan once if your broiler browns unevenly.
Carefully remove the baking sheet from the oven and let the grapefruit cool for 3 to 5 minutes. The topping will set up slightly as it cools, giving you that dessert-like brulee feel.
Serve warm, scooping through the caramelized topping and into the grapefruit segments with a spoon. Encourage everyone to hold the grapefruit steady with one hand or a towel if the peel is still hot.
For picky eaters or kids, you can dial back the cinnamon to 1/2 teaspoon and add a small drizzle of heavy cream or a spoonful of whipped cream on top after broiling to make it extra dessert-like. If your family likes a nutty crunch, sprinkle finely chopped pecans or walnuts over the caramelized mixture before broiling, keeping in mind this will add a few extra calories but still stay low carb. For a warmer, cozier flavor, add a pinch of nutmeg or pumpkin pie spice to the mixture along with the cinnamon. If you prefer a stronger sweet flavor, use an extra tablespoon of brown sugar substitute, tasting the mixture before spreading it on the grapefruit. To cut the bitterness of grapefruit for those who aren’t big fans, choose ruby red or pink grapefruits, which are naturally sweeter, and add a tiny extra pinch of sea salt to balance the flavors. You can also prepare the grapefruit the night before by halving and loosening the segments, then in the morning just mix the caramelized topping, spread it on, and broil for a quick weekday treat. For anyone watching dairy, swap the butter for melted coconut oil; it will give a light coconut note that pairs nicely with the citrus while keeping the recipe low carb.
