Puree 1 tomato, 1/2 serrano chile and 1 cup cubed seedless watermelon in a blender. Add 1 teaspoon red wine vinegar and 1/4 cup olive oil and pulse. Add 2 tablespoons minced red onion, 1/2 chopped seeded cucumber and 2 tablespoons minced dill; season with salt and pepper and puree. Chill at least 30 minutes. Loosen with water, if needed. Garnish each serving with more cubed watermelon, crumbled feta and dill.