Start by making your souvlaki by putting your lamb chunks in a big bowl, adding all of your other souvlaki ingredients but the duck fat. Stir to coat your meat chunks in the marinade before covering with plastic wrap and refrigerate for at least 2 hours, up to 24 hours.
After the lamb chunks have marinaded appropriately, thread the chunks onto your skewers, roughly 4 per skewer. If you have leftover marinade, you can save it for a future batch (store in the refrigerator for up to several days.) Heat a large frying pan or griddle pan with your fat over medium-high heat, adding the skewers in a single layer once hot. Cook 8-10 minutes, turning regularly with a fork until cooked through and browned on all sides. You can also use an outdoor grill.
At the same time, you can prepare your tzatziki. Start by grating your cucumber into a bowl, adding your yogurt, garlic, lemon juice, lemon zest, olive oil, and dill, salting and peppering to taste. Mix until combined, serving immediately with your skewers.