1) Preheat broiler to high heat.
2) In a small bowl, stir together sugars, salt, red pepper flakes, and rosemary.
3) Cut grapefruits in half and use a serrated knife to cut around the edges of the grapefruit and into each segment to separate the fruit from the grapefruit membrane.
4) Divide the topping between the four grapefruit halves. Pack it on each half.
5) Add grapefruit halves to a baking sheet and broil for 2-3 minutes until sugar is starting to caramelize and grapefruit is hot.
6) Remove, let cool briefly, but serve while still slightly warm.