Cut the watermelon in 1/2 inch slices, as large as possible. Prepare the remaining vegetables, slicing or mincing as indicated.
Using a lidded casserole dish as a “mold” cut the watermelon to fit in an even layer. Puzzle piece slices together as necessary.
Layer on a few slices of red onion, summer squash, cucumbers, arugula or spinach, sprinkle with minced jalapeno and finish by pressing in the slices of cotija or feta cheese. Repeat with another layer of watermelon. Another layer of the filling. Repeat until the ingredients are used up or your mold is full. Finish with the watermelon to seal everything in.
Place a plate or lid, just smaller then the width of the casserole dish on top of the final layer of watermelon. Top with a weight such as a couple cans of food. Place in the fridge.
Press for at least 15 minutes up to overnight.
When it is time to serve, carefully pour off most of the excess juices. Whisk a couple tablespoons of this juice with the dijon mustard, lime zest & juice. Slowly drizzle in the olive oil while whisking. Flavor with salt and pepper to taste.
Place a larger platter or tray on top of the casserole. Flip over on to the tray. Remove the casserole ‘mold’ and drizzle on the dressing. Top to finish with the arugula.