Combine the bulgur and boiling vegetable broth in a medium bowl; cover and let sit until tender, about 20 minutes. If you used water, stir in 1/2 teaspoon of salt to mixture.
Drain the bulgur in a fine mesh sieve. Set aside. (I skipped this step as my bulgur was pretty dry.)
Preheat oven to 400F. Line a 9×13 pan with foil and place the peppers (cut side up) in the pan. Set aside.
Heat 2 tablespoons of oil in a large skillet over medium heat; saute the onions until softened, about 5 to 7 minutes.
Add the mushrooms and cook for 2-3 minutes. Next add garlic and cook for an additional minute.
Stir in spinach and 1 tablespoon of water and cook (covered) until wilted, about 3 to 5 minutes.
Remove from heat and stir in parmesan cheese and salt and pepper to taste.
Mix the bulgur and spinach mixture together and spoon mixture into the peppers. Pour 4 tablespoons of water into the bottom of the pan; cover with foil or lid and bake until the peppers are tender, 30-45 minutes, depending on how soft you want the peppers.
Coat a small skillet with non-stick spray and heat over medium heat. Add the pine nuts and cook until golden brown, about 1 to 2 minutes, stirring constantly.
Sprinkle the toasted nuts on top of the stuffed peppers and serve.