Add the sliced red onion to a large mixing bowl. Pour the orange juice over the onion and toss well so the onion is coated and mostly submerged.

Place a metal colander over the bowl. Add the quartered tomatoes to the colander and sprinkle with 2 teaspoon sea salt. Toss gently and let sit for 30 minutes so the tomato juices drain into the bowl below.

In a small bowl, combine the remaining ½ teaspoon sea salt, onion powder, garlic powder, Italian seasoning, and black pepper. Stir to combine. Set aside.

Preheat the air fryer to 400°F and set the cook time to 4 minutes.

Brush both sides of the ciabatta slices with olive oil. Sprinkle generously with the seasoning mixture on both sides. Reserve ½ teaspoon of the seasoning blend for the vinaigrette.

Place the bread slices in the air fryer basket. Cook for 2 minutes, flip, then cook for another 2 minutes until crisp and lightly golden.

Remove the colander from the bowl. Reserve ¼ cup of the orange-tomato juice mixture for the dressing. Add the softened onions and salted tomatoes to a clean large bowl.

Prepare the vinaigrette by combining the reserved orange-tomato juice, white balsamic vinegar, olive oil, honey, reserved seasoning mixture, and Dijon mustard in a jar with a lid. Shake vigorously until combined.

Add the watermelon, cucumber, basil, and 2 ounce feta cheese to the bowl with the tomatoes and onion. Toss gently to combine.

Tear the toasted ciabatta into bite-sized pieces and add to the salad just before serving. Drizzle with some of the vinaigrette and toss gently. Transfer to a serving platter or bowl, then top with the remaining feta cheese. Serve immediately with extra vinaigrette on the side.