To sweat the tomatoes: Place a colander inside a medium bowl. You want to reserve the juices from this process. Add the tomatoes to the colander and toss with half of the sea salt. Allow to “sweat” for 30 minutes.
Preheat the oven to 425 degrees.
Cut the rustic loaf into 1-inch cubes. Spread the bread cubes on a rimmed baking sheet lined with parchment paper.
Drizzle with 3 T olive oil and sprinkle with remaining sea salt. Toss to combine.
Bake for 15 minutes until the cubes are crisp and browned at the edges. Cool completely.
Make the vinaigrette: In a medium jar with a tight fitting lid, add the tomato juices, Dijon mustard, Champagne vinegar, olive oil, salt and pepper. Close with the tight fitting lid and shake vigorously until the vinaigrette has emulsified.
In a large bowl, add the tomatoes, cucumber, mozzarella and red onion.
Add half of the vinaigrette and toss to combine.
Add the bread cubes, watermelon, basil, oregano and remaining vinaigrette. Season with salt and pepper to taste. Toss to combine.
Let sit for 10 minutes. Give it a good toss just before serving. I usually transfer my salad to a large platter to serve.