In a large saucepan cook potatoes, covered, in a small amount of boiling water for 25 minutes or until tender. Drain potatoes.
Preheat oven to 425 degrees F.
Mash drained potatoes with a potato masher or beat with an electric mixer on low speed.
Add yogurt, margarine or butter, salt, and red pepper; beat just until combined.
Add eggs; beat with an electric mixer on low speed for 1 minute or until fluffy.
Fold in 1/2 cup of the feta cheese.
In a 12-inch skillet bring the 1/4 cup water to boiling; add spinach to skillet. Toss spinach for about 1 minute or until spinach is slightly wilted; drain. Press out excess liquid. Coarsely chop spinach.
Spread half of the potato mixture into a lightly greased 2-quart rectangular baking dish. Layer spinach on top; sprinkle with remaining feta cheese. Spread remaining potato mixture on top and sprinkle with paprika.
Bake, uncovered, for 15 minutes or until top is lightly browned.