Combine the lamb, feta, oregano, ½ teaspoon salt, and ¼ teaspoon pepper. Divide the mixture into 4 equal portions. Form each portion loosely into a ¾-inch thick patty. Using your thumbs, make a deep depression in the center of each patty. Brush both sides of the burgers with a little olive oil and season with a little extra salt and pepper.

If there is a woody part of the stem in the mushroom caps, cut it out with a paring knife. Brush both sides of the mushroom caps with olive oil and season with a little salt and pepper.

Heat a grill to high. Place the mushroom caps on the grill, gill-side up. Grill for 1½ – 2 minutes. Flip and grill the other side for another 1½ – 2 minutes. Remove the mushrooms from the grill and keep warm.

Reduce the heat of the grill to medium. Place the lamb burgers on the grill and cook, covered, for 3-4 minutes. Flip and cook, covered, another 3-4 minutes or until the burgers are firm to the touch. Place the cooked burgers on top of the mushroom caps and top each with a dollop of Quick Tzatziki Sauce.

If using a grill pan: Heat the grill pan on top of the stove over high heat until a drop of water sizzles immediately when splashed on the pan. Cook as directed above.