In a large bowl, combine thinly sliced Fresh Mint Leaves (2 tablespoon), Scallion (1), and Dijon Mustard (1 tablespoon). Season with Salt (to taste) and Freshly Ground Black Pepper (to taste).
Add the Ground Beef (6 ounce) and Ground Lamb (6 ounce) and mix together until everything is just combined. Form into 4 patties, or 6 smaller patties (as long as it’s an even number).
Press the Feta Cheese (2 ounce) into half of the patties, and place the other half on top, covering the cheese. Gently pinch and smooth all the way around the edges of the burgers to seal the cheese in the middle.
In a medium skillet over medium heat, melt the Butter (1 tablespoon). Add the burger patties and cook each side for 4 to 5 minutes, or until your preferred doneness.
Serve patties on a hamburger bun, in a pita, on top of a salad, or in lettuce wraps. Garnish with additional mint if desired.