Step 1: Wash tomatoes thoroughly. Core them and, if desired, peel by blanching in boiling water for 30-60 seconds then transferring to an ice bath. Rough chop the peeled tomatoes.
Step 2: In a large (16-20 quart) stockpot, heat olive oil over medium heat. Add chopped onions and sauté until softened and translucent, about 7-10 minutes.
Step 3: Add minced garlic and cook for another 1-2 minutes until fragrant. Be careful not to burn the garlic.
Step 4: Add the chopped tomatoes to the pot. Stir in dried oregano, dried basil, red pepper flakes (if using), black pepper, and salt. Bring the mixture to a rolling boil, then reduce heat to a gentle simmer.
Step 5: Simmer the sauce uncovered for at least 2 hours, or until it has thickened to your desired consistency. Stir occasionally to prevent sticking. If you prefer a smoother sauce, use an immersion blender directly in the pot, or carefully blend in batches using a regular blender (vent the lid!). Add fresh parsley and fresh basil during the last 20 minutes of simmering.
Step 6: While the sauce is simmering, sterilize your canning jars and prepare lids according to manufacturer instructions. Keep them warm.
Step 7: Add 1 tablespoon of bottled lemon juice or 1/4 teaspoon of citric acid to each hot pint jar, or 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid to each hot quart jar. This is essential for safe water bath canning.
Step 8: Carefully ladle the hot sauce into the prepared jars, leaving 1/2 inch headspace (the space between the sauce and the rim of the jar).
Step 9: Use a non-metallic utensil (like a plastic spatula or chopstick) to remove any air bubbles from the jars. Gently press it around the inside of the jar.
Step 10: Wipe the rims of the jars clean with a damp cloth to ensure a proper seal. Center warm lids on the jars and apply screw bands finger-tight (do not overtighten).
Step 11: Place the filled jars on the rack in a water bath canner filled with hot water, ensuring jars are covered by at least 1-2 inches of water. Bring the water to a rolling boil.
Step 12: Once boiling, process pint jars for 35 minutes and quart jars for 40 minutes. Adjust processing time for altitude if you live above 1,000 feet (consult a reliable canning guide for specific adjustments).
Step 13: After processing, turn off the heat and let the jars sit in the canner for 5 minutes before carefully removing them with canning tongs. Place jars on a towel-lined surface, spaced apart, and do not disturb for 12-24 hours.
Step 14: After 24 hours, check seals. Lids should be concave and not flex when pressed. Remove screw bands, wipe jars clean, label with date, and store in a cool, dark place. Refrigerate any unsealed jars and consume within one week.