In a food processor, process celery, onion, garlic, sage, marjoram, and sea salt (if desired). Process until onion and celery are finely chopped but not mushy.
In a medium mixing bowl, mix turkey with egg.
Add celery mixture to turkey and mix until well blended.
Form turkey into six patties.
Heat oil in a large skillet over medium heat.
Cook turkey patties in batches for about 5 minutes each side, or until cooked through and browned on the outside.
Remove patties from skillet and place on a plate as the rest finish cooking. Cover to keep warm.
Double lettuce leaves and place on plates.
Place turkey burgers on lettuce wraps and add condiments as desired.
Top with tomato and avocado. Place Vegenaise and mustard next to each burger.