Trim the ends off 8 medium zucchini and shred them using a food processor or box grater. Toss the shredded zucchini with 2 tablespoons pickling salt in a colander and let sit for 10 minutes to draw out excess moisture.

Press the zucchini gently with a clean towel or your hands to remove liquid, then transfer the drained zucchini to a large 8-quart stainless steel pot.

Dice 3 medium onions and 1 red and 1 green bell pepper into small uniform pieces, and mince 2 garlic cloves.

Add the diced onions, bell peppers, minced garlic to the pot with zucchini. Pour in 2 cups apple cider vinegar, add 1/4 cup sugar, 2 teaspoons yellow mustard seeds, 1 teaspoon turmeric powder, 1/2 teaspoon celery seeds, 1/4 teaspoon cayenne pepper, 1/4 teaspoon black peppercorns, and squeeze in juice of 1 lemon. Stir to combine.

Place the pot over medium-high heat and bring to a rolling boil, stirring occasionally, about 5 to 8 minutes.

Reduce heat to medium-low and simmer uncovered for 30 minutes, stirring occasionally to prevent sticking.

Taste the relish and check that the vegetables are tender but still hold shape and the liquid has reduced by about one-third. Adjust seasoning with salt, vinegar, lemon juice, or sugar if needed.

Remove from heat and let cool to room temperature, about 30 minutes. Transfer to sterilized jars and refrigerate for immediate use or process in a boiling water bath for 10 minutes for shelf-stable canning.