Bring a large pot of water to a boil over high heat. Add the pasta and cook according to package directions for al dente, stirring occasionally. Drain the pasta, reserving about a cup of the pasta cooking water.

While the pasta is cooking, warm the olive oil in a large deep-sided skillet over medium-high heat. Add the onions and cook, stirring constantly. If the onions begin browning, lower the heat and stir in a teaspoon of water.

When the onions have softened, add the garlic. Stir and cook until fragrant, about a minute longer, then add the mushrooms. Stir to coat and then spread them out across the pan.

Cook, stirring occasionally, until the mushrooms begin to soften. Sprinkle with salt, pepper, and red pepper flakes. Raise the heat under the mushrooms to high, and pour in the chicken stock or wine. Let it simmer and reduce for about 2-3 minutes, stir in the butter.

When the butter has melted, remove from the heat and add the spinach. Toss to wilt the spinach and then add the cooked pasta to the pan. Toss gently to coat. Add a splash or two of the reserved pasta water, only if needed. Sprinkle with parsley.

Scoop into bowls and top each serving with Parmesan cheese.