Heat butter in a skillet over medium to medium-low heat. Add sliced onion and sauté until golden brown and caramelized. Reduce heat if browning too fast.
Add sliced mushrooms and sauté until tender.
Place crust on a pizza pan or other baking pan. Spread pizza sauce, bread spread or pesto over prepared pizza crust. Evenly spread the onion, mushrooms and feta cheese over crust. Bake for 8-10 minutes or until the crust gets crispy and the cheese begins to melt.
In the mean time cook the spinach in the sauté pan by adding a little water and cooking over medium heat. Remove from heat as soon as the leaves wilt. When pizza is removed from oven, drain spinach and add to top of pizza. Cut and serve immediately.