Combine the lamb, veal, oregano, salt and pepper in a large bowl and mix to incorporate. Form into thirty 2-ounce patties.
Place the onions in a heat-proof bowl. Combine the vinegar, sugar, peppercorns, bay leaves, 1/4 cup salt and 1 cup water in a saucepan and bring to a boil. Pour the pickling liquid over the onions and let sit for at least 30 minutes.
Combine the mayonnaise, cumin, lemon juice, salt and pepper in a small bowl and mix to incorporate.
To serve: Prepare an outdoor grill or grill pan to high heat.
Grill the sliders to desired doneness, about 4 minutes per side for medium. Spread the bottom of each bun with the cumin mayo; top with some onion, feta and a slider, add the top of the bun and skewer with a sandwich pick.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Slainte Irish Pub, Baltimore, MD