We microwave the squash on high power for 2 minutes. We rotate the squash (carefully—it gets hot!) and continue to cook, checking in 2 minute intervals for a total of 8 minutes.
The squash is removed from the microwave when the shell is getting soft. We set it aside to finish cooking and to give it some time to cool so we can handle it.
After 5 minutes or so, we cut the squash in half, crosswise. With a spoon, we scoop out the seeds and fibrous flesh and discard. Holding the squash flesh-side down over a bowl, we pull the spaghetti-like strands of cooked squash out of the shells with a fork.
In a microwave-safe cup, we combine the oil and sage leaves. We heat in the microwave for 20 seconds and pour over portions of the cooked spaghetti squash. At the table, we sprinkle the squash with parmesan cheese.