Add 2 tablespoons of olive oil and the diced onion in a medium saucepan and place over medium heat
Cook the onions until they start to soften (reduce the heat if they begin browning)
Add green peppers, tomato paste and garlic. Mix to combine and cook until the peppers soften and the mixture begins to fall apart and dry out a bit
Add beef broth, rosemary, red pepper flakes, tomatoes and a pinch of salt and pepper
Mix to combine all of the ingredients and bring the mixture up to a simmer
Once simmering, reduce the heat to the lowest possible setting that it will continue simmering at
Simmer for 60 minutes, stirring occasionally to ensure nothing is sticking to the bottom of the pan
Once the sofrito is thick and reduced, transfer the pan to a heat proof surface
Use immediately or allow the sofrito to cool, transfer it into an air tight container and store in the refrigerator