I got this recipe (which comes from Barbados) several years ago. I like to make this as a fruit serving when we have a spicy Caribbean dish for dinner, because the succulent citrus rounds nicley complement the savory Caribbean spice blends. But I think it would also be great with other spicy cuisines as well (Mexican, Spanish, Thai, etc.). It's very simple to prepare, and it uses only 4 common ingredients. I hope you like it.
Peel both oranges and the grapefruit; discard the peels. Slice all the citrus fruits into thin rounds (about 1/4" thick). Place the citrus rounds on an ungreased baking sheet, overlapping the edges just a bit.
In a small bowl, combine the brown sugar and cinnamon and stir until thoroughly blended. Place about a teaspoon of the sugar mixture on the top of each citrus round. Using the back of the spoon, spread the sugar mixture evenly over the top of the citrus rounds.
Bake in the oven (at 400°) for about for 5 or 7 minutes, or until the fruit slices are heated through. Serve immediately, while the fruit slices are still warm.