1. 1. Preheat oven to 400. Bake potatoes about 50 minutes (until fork tender).
2. 2. About ten minutes before potatoes are done, heat olive oil in large skillet. Add mushrooms. Cook until tender. (about 5 minutes) remove to large bowl.
3. 3. Add more olive oil to skillet if needed. Add spinach (drained and defrosted) Heat through. Stir in vinegar.
4. 4. Add spinach to mushrooms. Add feta. Feta will melt a little.
5. 5. Cut an X in the potatoes. Fill X with spinach, mushroom, feta mixture.