Cook the shrimp and garlic in butter or olive oil in a pan over medium-low heat. If you have a sous vide, you can toss it all in a Ziploc bag and program your sous vide.
Once cooked, transfer to a bowl. Refrigerate until cooled, about one hour.
While shrimp is cooling, chop red onion/shallot, celery, and parsley. Mix chopped ingredients with mayo.
Chop cooled shrimp. Toss in mayo mixture to combine. Add salt and pepper to taste.
Split mixture between rolls. The number of sandwiches made will depend on the size of your rolls.