Preheat your oven to 400°F (200°C).

Wash and core your tomatoes. Cut them in half or quarters, depending on their size.

In a large bowl, toss the tomatoes, diced onions, and garlic with the olive oil, balsamic vinegar, sugar, salt, pepper, and red pepper flakes (if using). Make sure everything is nicely coated.

Spread the mixture in a single layer on two large baking sheets. Don't overcrowd the pans, or the vegetables will steam instead of roast.

Roast for 45-60 minutes, or until the tomatoes are soft and slightly caramelized, and the onions are tender. Stir halfway through for even roasting.

Once the vegetables are roasted, transfer them to a large pot or Dutch oven.

Bring the mixture to a simmer over medium heat.

Use an immersion blender to blend the sauce until smooth. Alternatively, you can carefully transfer the mixture to a regular blender in batches, being mindful of the hot liquid.

Stir in the chopped fresh basil and oregano.

Simmer the sauce, uncovered, for another 30-60 minutes, or until it has thickened to your desired consistency. Stir occasionally to prevent sticking.

Taste and adjust seasonings as needed. Add more salt, pepper, or sugar to taste.

Gather your canning supplies: a large stockpot with a rack, canning jars with lids and bands (make sure the lids are new!), a jar lifter, a wide-mouth funnel, and a ladle.

Wash the jars and lids in hot, soapy water. Rinse well.

Sterilize the jars by placing them in a boiling water bath for 10 minutes. Keep the jars hot until you're ready to fill them.

Sanitize the lids by simmering them in hot water for 10 minutes.

Add the bottled lemon juice to the sauce. Stir well.

Using the wide-mouth funnel, ladle the hot sauce into the hot jars, leaving 1/2 inch of headspace.

Remove any air bubbles by gently tapping the jar on the counter or using a non-metallic utensil.

Wipe the rims of the jars clean with a damp cloth.

Place the lids on the jars and screw on the bands fingertip-tight (not too tight!).

Place the jars on the rack in the boiling water bath canner. Make sure the jars are covered by at least 1 inch of water.

Bring the water to a rolling boil and process the jars for the recommended time, which depends on your altitude and jar size. For pints, process for 35 minutes. For quarts, process for 40 minutes. If you live at a higher altitude, you'll need to increase the processing time (check a reliable canning resource for specific recommendations).

Once the processing time is complete, turn off the heat and let the jars sit in the hot water for 5 minutes before carefully removing them with the jar lifter.

Place the jars on a towel-lined surface and let them cool completely, undisturbed, for 12-24 hours.

After the jars have cooled, check the seals by pressing down on the center of the lid. If the lid doesn't flex or pop, it's properly sealed.

If any jars didn't seal, you can either reprocess them with new lids within 24 hours or store them in the refrigerator and use them within a week.

Label the sealed jars with the date and contents and store them in a cool, dark place for up to a year.