Juice the grapefruits to produce 24oz of pulp-free juice.
NOTE: I put a fine strainer over the top of a wide-mouth mason jar to strain out any pulp or seeds from the juice before it went into the jar.
For the simple syrup, add the water and monk fruit sweetener into a small saucepan over medium heat. Stir until the sweetener dissolves, about 2 minutes. Then remove it from the heat and allow to cool. Once it's cool, add it to a mason jar or container to keep it in your fridge for up to a month!
Next, combine 4oz of the simple syrup with the 24oz fresh grapefruit juice. Give it a stir and a quick taste test. If you prefer it sweeter, add in .5oz at a time until it is sweet enough for you.
Finally, combine 7oz of the sweetened grapefruit juice with vodka of your choice over ice (standard liquor pour is 1.5oz per serving, but I'll leave that part up to you). Garnish with a fresh sprig of rosemary and a slice of grapefruit!
Serving Size 7oz