Preheat the oven to 375° F.
Melt the butter in a cast-iron skillet, add the spinach, and cook it quickly over fairly high heat, stirring, until it is just limp.
Remove from heat, add remaining ingredients, and mix well.
Pat into a buttered 9-inch pie pan. Use a fork first, then your fingers, to mold the crust. Pre-bake for 15 minutes at 375° F.
Preheat the oven to 350° F.
In a large, heavy skillet, cook the onions in butter with salt, until the onions are soft (5 to 8 minutes).
Add the mushrooms and the lemon juice. Cook, stirring, over medium heat, about 5 to 8 more minutes.
Gradually, as you stir, sprinkle in the flour. Keep cooking and stirring over medium-low heat another 8 to 10 minutes. Remove from heat.
Beat together the egg, yogurt, pepper, cheddar, and parsley. Stir the mushroom saute into this mixture, then beat it well.
Turn the filling into the pre-baked crust (you don't need to cool the crust first). Top with extra cheese and paprika.
Bake for 30 minutes at 350° F.
