Heat the olive oil in a small Dutch oven over medium heat.
Add the onion and cook, stirring occasionally, until softened, about 10 minutes.
Add the minced garlic and crushed red pepper flakes and cook, stirring constantly, for 1 minute.
Add the blended can of tomatoes to the Dutch oven. Simmer for about 20-30 minutes, or when the sauce has thickened.
Remove from the heat. Taste and season with sea salt and freshly cracked pepper, to taste along with a pinch of sugar if it needs it.
Bury the whole sprigs of basil in the sauce and set aside to allow the flavors to mingle.
Cook the spaghetti al dente in salted boiling water per package instructions. Drain the spaghetti, reserving 1/2 cup of the pasta cooking water.
Discard the basil sprigs and return the Dutch oven to the stove over medium-high heat.
Add the drained pasta along with reserved pasta water, if needed, and cook, stirring constantly, for 2-3 minutes.
Remove from the heat and add butter and parmesan cheese; toss until cheese melts.
Pour into the serving bowl and top with fresh basil. Serve immediately with additional fresh parmesan on the side.