Step 1

Heat the oil in a large pot over medium low heat. When warm, add the smashed garlic cloves and let fry for about 1 minute until very lightly golden. Add the oregano, 1/2 tsp salt, chili flakes, basil, and onion. Stir to combine.

Step 2

Immediately add the tomatoes and their juices, cover the pot, and turn the heat up to medium and bring to a simmer. Turn the heat down to maintain a low simmer and let cook for 45-60 minutes, stirring occasionally. Taste and add salt if needed.

Step 3

Remove the basil, then using an immersion blender (or transfer to an upright blender) pulse the sauce until the onion is blended into the sauce. At this point the sauce can be used immediately or over the next week.

Step 4

Sauté the spinach separately: heat a skillet over medium heat and add the teaspoon of oil. Add the spinach and sauté until wilted but bright green, about 2-3 minutes. Add the red wine vinegar and let cook another 30-60 seconds. Add the spinach to the sauce and let it cook in the sauce for 5 minutes. This can happen anytime after the sauce is done, either immediately or when reheating the sauce if made in advance.