Bring a large pot of generously salted water to a boil. Cook the spaghetti according to package directions until al dente. Before draining, scoop out at least 1 cup of pasta water and set it aside. Drain and set the pasta aside.

While the water heats, melt the butter and olive oil together in a large skillet over medium heat. Add the sliced onion and a pinch of salt. Cook, stirring occasionally, for 12 to 15 minutes until the onion is soft, golden, and starting to caramelize.

Reduce the heat to medium-low. Add the minced garlic and red pepper flakes directly to the onions. Stir constantly for 60 to 90 seconds until the garlic is fragrant and lightly golden — don't walk away here.

Pour in the heavy cream and stir to combine with the onion and garlic mixture. Let the cream simmer gently for 3 to 4 minutes, stirring occasionally, until it has reduced slightly and coats the back of a spoon.

Add the grated Parmesan in two additions, stirring between each, until fully melted and incorporated into the sauce. Season with salt and black pepper.

Add the drained spaghetti directly into the skillet. Toss well using tongs to coat every strand in the sauce. Pour in the reserved pasta water a little at a time — start with ¼ cup — tossing as you go until the sauce reaches your preferred consistency. It should be loose and glossy, not thick and gloppy.

Plate immediately. Finish with extra Parmesan, a scatter of fresh parsley, and flaky salt if you have it.