In a large bowl, mix breadcrumbs and milk. Let sit until absorbed.
Heat olive oil in a pan over medium heat. Cook onion with a bit of salt until soft, about 7 minutes. Add garlic and cook briefly. Let cool.
Add onion mixture to breadcrumbs, then mix in mayo, parsley, salt, pepper, and cayenne.
Once cooled, mix in egg and ground chicken gently.
Form into 6 patties using slightly oiled hands. Chill for 20 minutes.
Cook the Juiciest Ground Chicken Burgers on a grill or skillet over medium heat for 6–7 minutes per side until fully cooked (165°F internal temp).
Rest for a few minutes before serving.