In a mixing bowl, whisk together marinade ingredients (chopped bell pepper, white wine vinegar, water, lemon juice, vegetable oil, oregano, thyme, salt, black pepper, ground mustard and sugar) until combined.
In a large ziplock bag, pour in marinade. Add onions and shrimp. Toss back and forth to coat shrimp. Place bag in a mixing bowl or dish and refrigerate for at least 2 hours (or overnight). Flip bag back and forth periodically to coat shrimp evenly.
Serve cold and garnish with extra lemon juice, black pepper or herbs. Enjoy!