Place the sliced red onion in a large bowl. Pour the orange juice on top and toss to combine, ensuring the onion is submerged in the juice. Place a metal colander over the bowl. Add the tomatoes to the colander and sprinkle with two teaspoons of sea salt. Gently toss to combine and set aside for 30 minutes.
Combine the onion powder, garlic powder, Italian seasoning, remaining sea salt, and black pepper in a small bowl and stir to combine. Set aside.
Pre-heat air fryer to 400°F and set the cook time to 4 minutes. While the air fryer is heating, brush the bread slices with the olive oil and sprinkle each side with the seasoning mixture from Step 2. Tip: Reserve one-half teaspoon of the seasoning mixture for the vinaigrette.
When ready, add the bread slices to the air fryer basket and close the lid. Turn the bread after 2 minutes to evenly brown on both sides. Note: Cook time will vary depending on the make and model of the air fryer, so check the bread while flipping, and adjust the final cook time accordingly.
After 30 minutes, remove the colander from the large bowl and carefully drain the liquid in the bowl to use in the vinaigrette. Place the salted tomatoes and softened red onion in a large bowl and set aside.
Prepare the vinaigrette by combining one-quarter cup of the reserved orange-tomato juice with the remaining vinaigrette ingredients in a container with a tight-fitting lid. Cover and shake vigorously to combine. Set aside.
Add the balled or cubed watermelon, sliced cucumber, fresh basil, and two ounces of crumbled Feta to the bowl with the seasoned tomatoes and red onion. Gently toss to combine and set aside.
To serve, tear or cut the toasted bread into bite-sized pieces and add to the salad bowl. Drizzle some of the vinaigrette on top and toss to combine. Transfer to a large serving bowl or platter. Top with the remaining Feta cheese and serve immediately with the remaining dressing on the side. Enjoy!