Cook the pasta according to package instructions until al dente.
While the pasta cooks, heat olive oil in a large skillet over medium heat.
Add diced onion and minced garlic to the skillet, sautéing until translucent.
Incorporate the cherry tomatoes into the skillet, cooking for about 3-4 minutes.
Add the spinach and red pepper flakes, stirring until the spinach is wilted.
Combine the cooked pasta with the veggie mixture, tossing to blend well.
Season with salt, pepper, and stir in the Parmesan cheese if using.
Serve warm, garnished with additional cheese or red pepper flakes as desired.