Place the rack in the lowest setting in the oven and Preheat to 400 degrees and lightly grease a 9x9 baking dish or sheet pan.
Heat a large saute pan over medium high heat. Add the olive oil and swirl to coat the bottom of the pan. Once the oil is shimmering and hot, add the onion and stir. Once the onions have softened, add the garlic and stir. When the garlic is fragrant, add the salt, pepper, Italian seasoning, and spinach. Stir well so that flavor of the onions and garlic permeate the spinach. Remove the pan from the eye.
Place the mushroom caps bottoms up on the baking dish or sheet pan. Fill each cap with the spinach mixture, top with the mozzarella cheese, and place the pan into the preheated oven for 10-15 minutes or until the cheese is melted and bubbly.
Scatter the sun dried tomatoes and pine nuts over the pizzas and serve hot.