Cook the pasta in boiling salted water according to the package directions; drain and set aside.
Heat the olive oil in a large, deep skillet over medium heat.
Add the zucchini and onions, season lightly with salt and pepper; sauté, stirring frequently, until still firm but just beginning to soften, about 3 to 4 minutes.
Reduce the heat to medium-low and add the garlic and crushed red pepper flakes (if using). Sauté for 1 minute, stirring frequently.
Add the spinach; sauté, stirring frequently, until the spinach wilts down.
Add the vegetable stock and tomatoes; cook, stirring frequently, for another minute.
Turn off the heat and add the cooked pasta and the parmesan cheese; stir until well-combined.
Taste and add salt and pepper, if needed.
Let it sit for 5 minutes, stirring occasionally, to allow the pasta to soak up any liquid left in the pan.
Give it one final stir and serve immediately. Sprinkle with additional parmesan cheese, if desired.
