Lay out the zucchini noodles and pat them dry with a paper towel.
Boil 8 ounces of thick spaghetti in a large pot of salted water.
Chop a red onion and set aside 3/4 cups.
Peel and mince two cloves of garlic.
Heat olive oil in a large pan. A deep skillet will work best.
Add the onion and garlic to the pan and cook on medium-high heat until brown.
Add the crushed red pepper flakes and stir, cooking over medium heat.
Place the zucchini noodles in a colander.
When the spaghetti is cooked, set aside one cup of the pasta water.
Drain the pasta over the zucchini noodles in the colander.
Add the pasta and the zucchini noodles to the skillet with the onions, garlic, and red pepper flakes.
Add the water and toss.
Sprinkle 1/2 the Parmesan over the pasta and zucchini and toss.
Add the rest of the Parmesan and toss one last time.
For added brightness, zest a lemon over the dish and toss.
Sprinkle with a bit of salt and taste. Always start with a pinch and add more if necessary.
Snip fresh parsley over the finished dish.