I also made this pepper relish recently and really liked it. You can incorporate some hot peppers into the mix if you want it spicy. This is for canning with a hot water bath method. If you have never done this, please make sure to look up a reliable tutorial, like this one.
Pepper Relish
adapted from Well Preserved by Mary Anne Dragan
6 cups of finely chopped sweet peppers, combination of colors. A few hot peppers can be included in the 6 cups for a spicy relish.
2 cups of finely chopped onion
2 cups of cider vinegar
1 cup of sugar (I used ¾)
2 tablespoons of mixed pickling spice (I did not have a pre-made blend, so made my own with coriander seed, bay leaf, a few cloves, a few black peppercorns, 2 dried chilies, and a tsp of mustard seed) in a cheesecloth bundle or a tea ball.
2 teaspoons of dried hot pepper flakes or substitute with fresh hot peppers
2 teaspoons of salt
Combine peppers and onions into a bowl, and pour boiling water over them. Let sit 5 minutes, then drain well.
Combine the remaining ingredients into a large pot, and bring to a boil. Add the vegetables, and stir to combine. Reduce the heat to medium, stirring occasionally for about 30 minutes.
Remove from the heat, take out the spice bag. Spoon the relish into hot sterilized jars, leaving ½ inch of head space. Release the air bubbles, wipe the rims of jars clean, seal according to manufacturer’s directions. Process the jars in a boiling water bath for 10 minutes. Please refer to this process here if you are not familiar with canning.
Other ways to preserve peppers
I freeze them. Just chop up raw peppers, and slide them into a freezer bag. That is it. They can be used in all kinds of stews, soups and braises over the winter.
I make a big pot of sautéed peppers and caramelized onions and freeze that in small bags. In winter, it can be turned into a tasty spread or dip by placing the thawed mixture in a blender with some walnuts, or cheese, or olives or dehydrated tomatoes.
I roast peppers, tomatoes, eggplants, onions and summer squash or zucchini with olive oil, salt, garlic and herbs at 400F for about 45 minutes. I freeze that mixture in freezer bags and then use it as a base for pasta sauces, soups, and a topping for pizza.