Greek Bison Burgers

Name: Greek Bison Burgers
Ingredients: 1 pound ground bison; ½ cup cooked spinach, squeezed dry; ½ cup crumbled feta cheese, preferably sheep’s-milk; 2 teaspoons chopped fresh dill; 1 teaspoon chopped fresh oregano; 1 teaspoon ground cumin; 1 teaspoon minced garlic; ¾ teaspoon kosher salt; ½ teaspoon freshly ground pepper; 3/4 cup nonfat or low-fat Greek-style plain yogurt, (see Note); 1 teaspoon freshly grated lemon zest; 2 teaspoons lemon juice; 1 teaspoon chopped fresh dill; 1 teaspoon chopped fresh mint; Pinch of kosher salt; Freshly ground pepper, to taste; 4 French rolls, or 4-inch pieces of baguette, preferably whole-wheat, split and toasted; 16 thin slices English cucumber; 8 slices vine-ripened tomato; 4 thin round slices red onion