Step 1

Place a rack in top third of oven; preheat to 450°. Cut lemon in half. Thinly slice one half into rounds and remove seeds; set other half aside for sauce.

Step 2

Whisk garlic, 2 tsp. chopped oregano, and ⅓ cup oil in a large bowl. Pat chicken dry with paper towels and transfer to bowl; add onions and lemon slices. Season with salt and pepper; toss to coat.

Step 3

Heat 2 Tbsp. oil in a large heavy skillet, preferably cast iron, over medium-high. Cook chicken (leave onion and lemon behind in bowl), skin side down, until deep golden brown and crisp, 5–7 minutes. Transfer to a plate (chicken won’t be cooked through yet).

Step 4

Cook onions and lemon slices in the same pan, turning occasionally, until onions have taken on some color and lemon slices have shriveled slightly, about 5 minutes.

Step 5

Arrange chicken in pan, skin side up; transfer pan to oven. Roast until chicken is cooked through and onions are softened but still hold their shape, 16–20 minutes.

Step 6

Meanwhile, stir yogurt, feta, and remaining 2 Tbsp. oil in a medium bowl to combine. Squeeze in juice from reserved lemon half and add 2 Tbsp. water; mix well. Season feta sauce with salt.

Step 7

Arrange tomatoes on a platter; season with salt. Using tongs, arrange chicken, onions, and lemon slices on top. Pour any pan juices over and top with oregano leaves. Serve with feta sauce alongside.