Heat olive oil over medium heat in a large non-reactive pot. Add diced onion, then cook, stirring occasionally, until softened and lightly browned. Add 2 cloves of chopped garlic, then cook.
Add whole peeled San Marzano tomatoes, crushing the tomatoes either with your hands or with a spatula. When the sauce begins to simmer, add basil leaves and chopped oregano leaves. Season with 1 tsp Kosher salt then simmer.
Taste, then adjust seasoning with added Kosher salt as needed. For a slightly sweet marinara sauce, stir in sugar. Serve immediately or store in jars in a refrigerator until needed.