Altering recipes is not always a good idea so hate to say yes to your question for just any recipe because it could change the density of the product.  
Below are two links to relish recipes I found on the National Center for Home Food Preservation.  The Dill relish has no added onions and the pickle relish only has a cup added so it would be ok to eliminate the onions and add an extra cup of peppers or cucumbers.

https://nchfp.uga.edu/how/can_06/dill_pickle_relish.html

https://nchfp.uga.edu/how/can_06/pickle_relish.html

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Nellie Oehler