Slow-Simmered Red Sauce

Preheat your oven to 300°F.
While your oven is preheating, heat the olive oil and butter in a large Dutch oven over medium heat until the butter is melted. Add the diced onion and cook for about 5 minutes until softened.
¼ cup extra-virgin olive oil, 4 tablespoons butter, 1 medium onion
Add the garlic and cook for another 2 minutes until fragrant but not browned. Add the red pepper flakes and oregano and stir for 1 minute.
8 cloves garlic, 1 teaspoon red pepper flakes, 1 tablespoon dried oregano
Add the crushed tomatoes, carrot chunks, and basil stem. Season lightly with salt and pepper and stir to combine. Bring to a simmer over medium-high heat.
4 28-ounce cans crushed tomatoes, 1 medium carrot, 1 large stem fresh basil, Kosher salt, freshly ground black pepper
Cover with the lid slightly ajar and transfer to a 300°F oven, or reduce to the lowest stovetop setting. Cook for 4 to 6 hours, stirring occasionally, until the sauce has reduced by about a third and deepened to a rich dark red.
Remove from heat. Discard the carrot chunks and basil stem. Add fish sauce if using. Season generously with salt and pepper, stir in the fresh herbs and a drizzle of olive oil, and serve.
1 tablespoon fish sauce, ½ cup minced fresh parsley or basil