Preheat oven to 350 degrees. Place brioche squares on a baking sheet, and brush with butter. Bake until golden and crisp, 8 to 10 minutes. Combine fennel seeds and 1 teaspoon each salt and pepper. Reserve 1/2 teaspoon of spice blend, and sprinkle the rest all over the lamb; let stand 10 minutes.
Preheat a large saute pan over medium-high heat. Add 1 tablespoon oil, swirl once to coat bottom of pan, and add lamb pieces, at least 1 inch apart. Cook, turning, until browned on all 4 sides, about 4 minutes total. Transfer to a cutting board, let rest 10 minutes, and cut into 1/4-inch-thick slices.
Whisk together yogurt, remaining oil, 1/4 teaspoon salt, and the lemon juice. Top each toast with 1 mint leaf, 1 lamb slice, 1/4 teaspoon yogurt, zest, and reserved spice blend.