• In a deep skillet, sauté the onions and garlic over medium-high heat until they're translucent, about 5 minutes.
• Add the tomatoes and cook, stirring frequently, for another 5 minutes.
• Add salt to taste.
• Reduce the heat, cover, and simmer for 20 minutes. Uncover and continue to simmer for 10 more minutes.
• If the sofrito is too watery, you might need to cook it longer until the liquid reduces and reaches the desired consistency.

For Canning

• Transfer the sofrito to sterilized 6-ounce Mason jars, ensuring there's about half an inch (a little more than a centimeter) of free space at the mouth of the jar. Make sure the rim of the jar is perfectly clean, as any particle can interfere with the sealing process. When capping the jars, place the lid first, followed by the screw-on ring.
• Fill a canning pot halfway with water to process the jars. Place the jars on the canning rack inside the canning pot. The jars should be covered with at least an inch (2.5 cm) of water.
• Cover the pot and set it over medium heat until it boils. Once the water starts boiling, let it boil for 20 minutes.
• After the jars have been processed, place them on a kitchen towel and let them cool completely. The lids should make a "pop-up" sound and appear slightly concave in the middle. This indicates that the jar has been vacuum-sealed. If any lid doesn't make the sound and isn't concave, you'll need to reprocess the jar, store it in the fridge, and consume its content within the next few days.
• Sealed sofrito jars should be stored away from light. They will last up to 18 months.