Preheat oven to 375°. Marinate feta: In a large bowl, whisk together 1/4 c. extra-virgin olive oil, Juice of 2 lemons, 1 clove garlic, minced, 1 tsp. dried oregano, and 1/2 tsp red pepper flakes. Season with pepper and add 1 c. cubed feta, tossing to coat. Cover and refrigerate for about 10 minutes while preparing other ingredients. Roast fish: Scatter 1 lemon, thinly sliced and 1 small red onion, sliced at the bottom of a large baking dish. Add 4 salmon fillets, skin side down, to baking dish. Season with salt and pepper and bake until opaque and flaky, 18 to 20 minutes. Meanwhile, make topping: Into the bowl with feta, add 1 c. quartered tomatoes or halved cherry tomatoes, 1/4 c. sliced kalamata olives, 1/4 c. chopped Persian cucumbers, 1/4 chopped red onion, and 2 tbsp. freshly chopped dill. Fold gently to combine. To serve: Plate salmon with lemon and red onion slices and top with feta mixture.