Be sure to squeeze/juice the grapefruit and lemon.
Add to a pot and gradually whisk in sugar. Bring to a simmer. *This is where you'd add the rosemary or sprig of thyme.*
Once you notice bubbling pour in the water and stir.
If you have an ice cream machine you would put your mixture into there once it cools down a bit. Get it to a slushy consistency and then place in the bread pan to freeze overnight covered in a freezer bag.
If you do not have an ice cream maker/machine, you whisk it up and place it in a bread pan and into the freezer, covered in a freezer bag overnight.
You can check on it after several hours but once it has fully froze over you can scoop it and serve.
Note that this would also be good placed into a champagne flute and pour champagne over top.