In a large bowl, stir together breadcrumbs and milk; let stand to absorb.
Heat oil in a large saucepan over medium heat, then cook onion with ¼ tsp salt until softened, about 7 minutes. Add garlic and cook until fragrant, about a minute more. Cool for a few minutes, then scrape into breadcrumb mixture.
Stir mayo into the breadcrumb mixture, along with parsley, remaining salt, pepper and cayenne (if using).
When mixture is cool to the touch, stir in the egg then the ground turkey. Use your hands to gently but thoroughly work the mixture together. Form 5 burger patties.
Air fry the turkey burgers in batches in a single layer at 375ºF for 6 minutes. Flip burgers and continue cooking 2-3 minutes, until internal temperature reaches 165ºF. You’ll need to do two batches. Let burgers rest for a few minutes before serving.
Video
Notes
Cook the garlic until it is just fragrant; don't allow it to burn and turn bitter.
The egg should be added in once the mixture has cooled down.
Handle the mixture gently and lightly but ensure the ingredients mix together thoroughly.
The patties will need to be flipped during their cooking process.