Combine the buttermilk, Worcestershire, dry mustard, and cayenne in a shallow nonreactive baking dish. Stir in the onion and garlic and add the turkey cutlets, turning to coat. Cover and refrigerate for at least 4 hours or overnight.
Preheat the oven to 350°. Spread the bread crumbs on a baking sheet and toast for about 5 minutes, until lightly browned. Transfer to a plate and season with salt and pepper.
Drain the turkey cutlets and discard the marinade. Coat the cutlets with the seasoned bread crumbs, shaking off the excess. Coat a large nonstick skillet with olive oil cooking spray and warm over moderately high heat. Add one-third of the turkey cutlets and spray them lightly. Fry the cutlets, turning once, until cooked through and golden, about 3 minutes per side. Transfer to a platter, cover loosely with foil, and keep warm. Wipe out the skillet and repeat the process with more olive oil spray and the remaining turkey cutlets.