Crush tomatoes using your hand, breaking into smaller chunks. Allow the tomatoes to sit and drain in the colander for 5 minutes. Reserve the tomato juice.
Heat a dutch oven over medium heat. Once hot, add the olive oil. When the olive oil begins to shimmer, add in the onions. Saute until onions are softened and lightly browned, 5 minutes.
Add the garlic, oregano, chili flakes (if using), salt, and pepper. Stir and cook until fragrant, 30 seconds.
Add the strained tomatoes and stir to combine. Increase heat to medium-high, cook and stir frequently, until the liquid evaporates, and the sauce begins to brown in the edges, 10 minutes.
Add in 2 ½ cups of the reserved tomato juice. Stir the sauce, scraping any browned bits from the pan. Reduce heat to a simmer, stir and cook until the sauce thickened about 10 minutes.
Stir in the chopped basil. Taste sauce and season with more salt, pepper, and chili flakes as desired.
Notes
Recipe Yield: 3 cups of sauce.
Serving Size: 1/4 cup (60ml)
If you want a less thick marinara sauce, add more tomato juice.
For an even thicker consistency, reduce the sauce for longer. Some tomato paste can also be used for faster thickening.