Mix together turkey, spinach leaves, leeks, yellow peppers and seasonings until thoroughly incorporated. Form into 12 small patties.
On a preheated grill, about 4 to 5 inches from the heat, grill patties on both sides, about 10 to 11 minutes until the internal temperature reaches 165 degrees F.
Place a small cheese slice on top of each burgers, continue to cook just until cheese melts.
Place sliders on the focaccia and top with a second slice of focaccia.