Heat a large pot over medium heat and add the olive oil. Add the diced onion and sauté until translucent. Add the minced garlic and cook for another minute.
Add the tomato paste and cook for 2 minutes, stirring often.
Add the diced tomatoes, tomato sauce, sugar, dried oregano, dried basil, and red pepper flakes.
Stir the mixture and bring it to a simmer. Allow to simmer on low for 1-2 hours, stirring often.
Remove from heat and add in fresh basil, fresh parsley, and butter. Stir until combined and the butter has melted.
Use an immersion blender to blend the sauce until smooth or serve it chunky.
Season to taste and store in mason jars or serve over gluten-free spaghetti.
Notes
For dairy-free/vegan options: use your favorite dairy-free or vegan butter, or just omit it.
Store leftovers in an air-tight container in the refrigerator for up to one week. Spaghetti sauce can badly stain plastic, so store this in an airtight stain-proof container or plastic storage bag instead.
To Freeze: Once the sauce has completely cooled place it in an air-tight freezer container or plastic freezer bag. If using a glass container make sure to check that it is freezer safe and leave room for the sauce to expand as it freezes. The sauce can be frozen for up to three months. Defrost the sauce in the refrigerator overnight, or in the microwave.