Bring a large pot of generously salted water to boil. Cook pasta according to package instructions for al dente. Drain pasta in colander. Transfer pasta back into the empty pot and toss with 3 TB olive oil; cover and set aside.While spaghetti is cooking, heat 1 TB olive oil in large skillet until oil is hot. Add garlic and onion, stirring until aromatic and browned, 2-3 minutes. Add garlic mixture to the pot of cooked spaghetti and stir to combine well. Add broth to loosen pasta as needed. Add parsley and parmesan cheese, tossing until well coated. Add kosher salt and freshly ground black pepper to taste.If using, sprinkle with desired amount of red pepper flakes and serve with fresh lemon wedges and extra parmesan cheese.