Wash zucchini, tomatoes, onion, and bell peppers. Finely chop all vegetables and gather in a large mixing bowl.

In a large pot, heat olive oil and sauté zucchini, tomatoes, and onion for about 10 minutes until softened and fragrant.

Add minced garlic and cook for an additional minute. Pour in vinegar, add sugar, salt, cumin, crushed red pepper flakes, and black pepper. Bring to a boil.

Reduce heat to low and let the salsa simmer uncovered for 20-25 minutes, stirring occasionally until thickened.

Fold in chopped cilantro if desired.

Ladle hot salsa into sterilized jars, leaving ½ inch of headspace.

Wipe jars' rims clean, seal with sterilized lids, and screw bands on until snug.

Process jars in boiling water bath for 15 minutes.

Let jars cool undisturbed for 12-24 hours before storing.