Are you sensing a theme here? It's canning and pickling season. Lots of fresh produce around just begging to be turned into wonderful pickles, preserves, and relishes. These are fairly easy to make and usually safe for home canning, just keep the kitchen and jars as clean as possible and use a good quality vinegar that says 5% acidity on the label.
This one is another good one for burgers and hot dog toppings. It's got a nice onion flavor that isn't too overpowering.
The summer we tried this recipe out, I got a 50lb bag of onions for about $15. Great price, great deal, but that was an enormous pile of onions. We made at least a dozen batches of this relish. The kitchen reeked of raw onion for a week. We sat around the table crying our eyes out as we peeled and chopped onions. Lots of bad jokes happened.
If you have a salad shooter grater type thing, use it for the onions. Just use a coarse grating blade. Makes it go much faster than trying to chop it by hand.
1 quart finely chopped onions
2 c. finely chopped sweet red peppers
2 c. finely chopped green peppers
1 c. sugar
4 c. vinegar
4 t. salt
Combine all ingredients and bring to a boil. Cook until thickened, about 45 minutes, stirring occasionally. Pack into hot jars and process to seal.
For canning, check out these books and sites:
USDA Guide to Home Canning (PDF download)
Ball (brand of canning jars) site
National Center for Home Food Preservation