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.

In a large microwave-safe bowl, combine sliced onion, vinegar, ½ tsp sugar (1 tsp for 4 servings), and 2 TBSP water (4 TBSP water for 4). Season with salt and pepper.

Cover with plastic wrap; microwave for 30-60 seconds. Remove plastic wrap and set aside to pickle, stirring occasionally, until ready to serve.

Open canned tuna and drain; place in a medium bowl.

Stir in mayonnaise, sour cream, and half the garlic powder. Taste and season with salt and pepper.

Stir a drizzle of olive oil and a pinch of garlic powder into bowl with softened butter. Lightly season with salt and pepper.

Spread garlic butter onto cut sides of ciabatta; place on a baking sheet. (TIP: For easy cleanup, line baking sheet with aluminum foil first.) Toast on top rack until golden brown, 5-6 minutes. TIP: You can use your toaster oven instead.

Slice garlic bread into four triangles each.

In a second large bowl, whisk together juice from one lemon wedge, half the mustard, 1 TBSP olive oil, and ¼ tsp sugar (for 4 servings, use juice from two lemon wedges, all the mustard, 2 TBSP olive oil, and ½ tsp sugar). Season with salt and pepper.

Add spinach, cucumber, and as much pickled onion as you like (draining first) to bowl with dressing; toss to combine.

Divide salad between bowls. Top with tuna and more pickled onion if desired. Serve with garlic bread and remaining lemon wedges on the side.