In a large bowl, combine the lamb with the shallot, garlic, thyme, pepper, and salt. Form 4 patties, each about 3/4 inch thick. Place on a medium-hot grill and cook 5 to 6 minutes per side. (To cook on a stovetop, heat the oil in a grill pan over medium heat and cook the patties 6 minutes per side.) Serve in the pita pockets and top with Cucumber-Mint Relish, if desired, and mint leaves to taste.