Turn on the grill from the oven and cut the grapefruit in half. If they are not straight, cut some of the peel off with a knife. Cut the flesh off the skin with a very sharp knife, but leave it in the fruit. Place the halves in a suitable baking dish. If they are still a little uneven, you can put some aluminum foil underneath them.
Melt the butter and sugar in a small saucepan and add the cinnamon. Now pour this mixture evenly onto the grapefruit halves. Smooth out and put in the oven. If possible, place 10-11 cm under the grill for about 2-3 minutes. The topping should bubble and have bubbles. You should look there so that they don`t get too dark.
Take out and serve immediately.
If you like, you can garnish it with a few mint leaves. The grapefruits should be chilled very well before use.