1
Adjust rack to top position and preheat oven to 450 degrees. (TIP: If you have a toaster oven, feel free to skip preheating—you’ll use it to toast the ciabatta in Step 4.) Place 1 tsp butter (2 tsp for 4 servings) in a small bowl to soften. Wash and dry produce.
Halve, peel, and thinly slice onion. Halve ciabatta. Quarter lemon. Trim and halve cucumber lengthwise; thinly slice crosswise into half-moons.