In a large bowl, whisk together vinegar, oil, mustard, garlic, salt, and red pepper. Stir in shrimp, onion, and bell peppers. Cover and refrigerate for at least 4 hours or up to overnight.
Strain shrimp mixture, reserving marinade. Serve over spring mix with reserved marinade.
Recipe by Louisiana Cookin' at https://louisianacookin.com/mardi-gras-marinated-shrimp-salad/