1. In a medium bowl, mix first 8 ingredients (through feta). Use an ice cream scoop or 1/2-cup measure to make equal-size patties.
2. Broil for 5–6 minutes on each side or until done.
3. Place burger on bottom half of bun. Top with onion slices and a Dijon-and-ketchup mixture (a trick for cutting sugar from your diet), if desired; garnish with parsley, if desired. Cover with top half of bun.