Cook the whole wheat pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of the pasta water.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until translucent.
Stir in the minced garlic and sliced mushrooms. Cook for an additional 5-7 minutes until the mushrooms are tender and have released their moisture.
Add the fresh spinach to the skillet, stirring until wilted, about 2-3 minutes.
Season the mixture with dried oregano, salt, and black pepper. If using, stir in the heavy cream and mix well.
Add the cooked pasta to the skillet, tossing to combine. If the mixture seems dry, add reserved pasta water a little at a time until desired consistency is reached.
Remove from heat and stir in the grated Parmesan cheese. Serve immediately, garnished with fresh parsley.