In the bowl of your food processor tear up one English muffin and pulse several times until very fine crumbs form.
Add half of the shrimp (reserving the other half for later), the egg white, the white parts of the green onions (reserving the green parts for later), cilantro, the zest of the lemon, Old Bay Seasoning, 1 tsp garlic powder and salt and pepper. Pulse until the mixture becomes thick and paste-like.
Pour the mixture into a bowl along with the remaining shrimp and panko breadcrumbs, stir to combine. Shape the mixture into 4 patties and place on a parchment lined plate in the fridge for at least 30 minutes.
Preheat grill to medium high heat and be sure to brush grates with olive oil to prevent sticking. Carefully place the patties on the grill and cook for 3 to 4 minutes per side. Remove from grill and keep covered until ready to serve.
To prepare the lemon sauce combine the mayo with the juice of the lemon, 1 tsp garlic powder, 1 tsp celery salt and 1/3 cup of diced green onions. Stir to combine.
Toast the reserved English Muffins until crisp and slather each side with the sauce. Top with the shrimp burger and any optional toppings you desire.