Take a heavy-bottomed saucepan and combine the tomatoes, garlic, onion, red pepper flakes, oregano, and olive oil.
Cook over high heat until the mixture turns to a simmer. Then, lower the heat and cook until you see the oil floating free of the tomatoes. Then using a sturdy wooden spoon, smash the tomatoes.
Once the tomatoes are cooked, remove the pan from the heat, mash the garlic cloves and onion, and stir well. If desired, take an immersion blender and blend the mixture as per your liking.
Add some salt to taste and serve warm.
The marinara sauce will remain unspoiled for 4 days if covered and refrigerated. If frozen, it will remain preserved for 6 months.
Notes
You can also use a can of tomatoes that is BPA-free. Always use high-quality tomatoes.