Heat olive-oil in a medium-pot over moderate heat. Add mushrooms & onion and simmer for 5-7 mins, until tender and the liquid from the mushrooms was largely cooked.
Insert garlic and season with pepper, salt and red-pepper flakes (if using) and simmer another 30 secs.
Increase heat to high then include canned tomatoes & vegetable broth. Bring to a boil.
Add spaghetti-noodles & stir until they wilt and fit in the pot. Cover the pot and reduce heat to cook & simmer for 5 mins. (Your liquid ought to be in a very low boil this entire moment.)
Remove lid & put in fresh-spinach, a couple handfuls at a time, stirring to acquire the lettuce blended in and simmer.
Remove from heat and Allow spaghetti sit, covered, for a Couple of Minutes
Stir in Parmesan-cheese, taste and correct seasonings.
Serve with fresh-basil or parsley and additional Parmesan-cheese, if needed.