Wash the potatoes well, then prick the skin several times with a fork. Place the potatoes on a microwave safe plate and microwave on high for 5 minutes.
Pierce the potatoes in the center with a fork to test for doneness. If they still feel crunchy in the center, microwave longer, one minute at a time, until they are tender in the middle (about 8 minutes).
Let the potatoes cool for just a few minutes. Slice the potatoes into several pieces to make them easier to mash, then transfer to a large bowl. Add the butter, garlic powder, salt, some freshly cracked pepper, and milk. Mash the potatoes until the butter is melted and the seasonings are evenly distributed.
Add the fresh spinach and stir it into the potatoes. Allow the residual heat of the potatoes to partially wilt the spinach. Leaving the spinach only partially wilted gives the potatoes more texture and volume.
Finally, crumble the feta cheese over top and fold it into the potatoes. Taste the potatoes and adjust the salt, pepper, or butter if needed.