In a large bowl, whisk together the flour, baking powder, salt, and dry milk. Mix in the butter with a fork, a pastry blender, a mixer, or your fingers, till crumbly.
Preheat the oven to 375°F.
Sauté the onion in the butter until it’s soft and becoming golden brown, about 10 minutes.
In the meantime, add the ricotta, salt, egg substitute or eggs, spinach, and grated cheese to the large bowl.
When the onions are almost done, stir in the thyme and nutmeg. Remove them from the heat, and add them to the large bowl.
Mix everything together thoroughly, and spoon it into a greased pie pan. Spread with the sour cream. This is best done by dolloping the sour cream atop the filling, then using a spatula, or the back of a spoon, to smooth down and connect the dollops. Sprinkle with paprika, if desired.
Bake the quiche for 50 to 55 minutes, or until it appears set almost all the way into the center, and has begun to brown slightly. It may crack, also; that’s OK.
Remove it from the oven, and cool it on a rack for at least 20 minutes, preferably 30, before serving.