In a medium bowl whisk together onion, garlic, cayenne, rosemary and 3 tbsp vinegar until well blended. Season with salt and pepper. Remove 2 tbsp of marinade and transfer to a small bowl; set aside. Add shrimp and tomatoes to the remaining marinade and toss to coat.
Heat grill to medium. Lay down a piece of foil and curve all sides upward. Pour shrimp mixture in the center of the foil. Close the lid and cook for about 8-10 minutes (until tomatoes look like skins are slightly bursting)
In a large bowl, toss spinach and tomatoes with reserved 2 tbsp marinade. Top with shrimp and tomatoes.
Notes
*Serving sizes on the recipes are not necessarily "bariatric sized" portions but are "traditional" portion sizes.