Combine the lamb, grill seasoning, fennel seeds, oregano, cumin and crushed red pepper. Form into 4 patties.
In a large skillet, heat the EVOO, 1 turn of the pan, over medium-high heat. Add the patties and cook, flipping once, for 3 minutes on each side for medium-rare.
Using a food processor fitted with a grating disk, grate the cucumber and garlic. Add the yogurt, feta and lemon juice and puree.
Spread some tzatziki sauce on the roll bottoms and top with the burgers, lettuce, tomato, red onion, more sauce and roll tops. Serve with the potato chips.