Step 1 Puree the onion in a blender. Squeeze the onion juice through a fine strainer or double layers of cheesecloth and discard the solids. You should have about one-half cup of onion juice. Mix the juice with the crumbs, allow to sit 2 to 3 minutes and then squeeze out excess juice.
Step 2 Place the crumbs, beef, salt and pepper in a food processor and process until it forms a ball on top of the blade, or knead thoroughly by hand. Form the meat into eight patties.
Step 3Grill the patties until medium rare, about 5 minutes per side. While grilling, mix the sour cream and mustard together. Serve the patties on the toast and top with 1 tablespoon of the sour cream-mustard sauce.