Combine peas, onion, water, and salt in a medium saucepan. Bring to a boil. Reduce heat and simmer, covered, according to package directions or until peas are tender. Drain, reserving liquid. Stir mint into peas.
Transfer pea mixture to a food processor. Process for several minutes until smooth, adding enough liquid to obtain a thick puree. Cover and chill for 30 minutes. Spoon chilled puree into a pastry bag fitted with a small tip. Pipe onto Melba toast squares or crackers.