Warm olive oil in a large, heavy-bottomed pot over medium heat. Once you see the oil shimmering, add the chopped onion and sauté for 1 minute, stirring occasionally. Add the minced garlic and pinch of red pepper flake then continue to sauté until onions are soft and translucent, stirring occasionally.
Add crushed tomato, tomato paste, oregano, basil, sugar, a few pinches of kosher salt and several turns of freshly ground black pepper, stirring to combine. I also like to fill the empty can of tomato paste with water then add that to the sauce (6 oz.). Bring sauce to a boil, reduce heat to low then simmer, partially covered, for 1 hour (1.5 to 2 hours is best if you have the time).
To finish the sauce, stir in a handful of chopped parsley. Season to taste with additional kosher salt and freshly ground black pepper.
Serve this sauce with your favorite pasta. I like to add two or three sizable spoonfuls of sauce to freshly drained spaghetti, tossing to coat. This will ensure that the spaghetti absorbs the flavor of the sauce. Place a mound of spaghetti on a plate then top with extra sauce (I like it saucy :)), chopped parsley, grated Parmesan cheese and freshly ground black pepper.