I'm not a huge fan of turkey burgers but a friend of mine showed me this method.
It's simple. Saute finely chopped celery, (a stick of so) for about 3 minutes in a tablespoon or so in olive oil, add some chopped mushrooms, (about a cup for 2 burgers), another 5 minutes or until they are soft. Add to ground turkey. Spice how you like, pepper, salt, Worcester sauce etc. Grill.
I've found these organic, panko crusted, non trans fat onion rings, (baked), that goes great with the burgers