10 cups zucchini or summer squash, finely chopped, grated, or shredded (about 4 pounds)

3 cups yellow or white onion, finely chopped or grated (about 2 medium onions)

3 cups red bell pepper, finely chopped or grated (about 3 large peppers)

2.5 cups vinegar (1.5 cups apple cider vinegar + 1 cup white vinegar, 5% acidity)

2.5 cups white cane sugar

4 tbsp pickling salt, sea salt, or kosher salt (not iodized table salt)

2 tsp whole yellow mustard seed

1 tsp celery seed

1 tsp ground turmeric

1/4 tsp nutmeg

1/4 tsp black pepper

Optional: 1–2 fresh hot chili peppers (finely chopped) or 1–2 tsp red chili flakes

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Wash zucchini, bell peppers, and onions. Trim zucchini ends and remove some seedy centers before cutting into chunks.
Finely chop zucchini, onions, and peppers (using a food processor or by hand). Measure 10 cups zucchini, 3 cups onions, and 3 cups peppers into a large non-reactive bowl.
Sprinkle vegetables with 4 tbsp salt, mix well, and refrigerate for several hours or overnight.
Rinse vegetables thoroughly in a fine-mesh strainer or cheesecloth under cold water. Press out excess liquid.
In a large non-reactive pot, combine drained vegetables with vinegar, sugar, mustard seed, celery seed, turmeric, nutmeg, and black pepper. Bring to a boil over medium-high heat, stirring occasionally.
Reduce heat and simmer for 25–30 minutes, stirring regularly, until the mixture thickens.
For canning: Sterilize jars. Fill with hot relish, leaving 1/2-inch headspace. Wipe rims, apply lids, and process in a boiling water bath for 15 minutes (adjust for altitude). Cool and check seals.
For freezing: Cool relish to lukewarm, transfer to freezer-safe containers, leaving room for expansion. Refrigerate overnight before freezing.
For refrigeration: Store in sealed jars in the fridge for up to several months. Allow flavors to develop for 1–2 weeks before use.

Use yellow summer squash instead of zucchini for a different flavor.
Add jalapeños or chili flakes for a spicier relish.
Adjust sugar to taste, but note it may affect texture and preservation.
Best flavor develops after 1–2 weeks.
Canned relish lasts up to 18 months in a cool, dark place; refrigerate after opening.