In a small bowl, combine the low sodium soy sauce, light brown sugar, whole grain mustard, and Cajun seasoning. Whisk until the sugar is dissolved and the mixture is well blended.
Place the peeled and deveined shrimp in a large mixing bowl. Pour the marinade over the shrimp, ensuring they are well coated. Let them marinate for about 15 minutes at room temperature.
In a large skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant, being careful not to let it burn.
Add the marinated shrimp to the skillet in a single layer. Cook for 2-3 minutes on one side until they turn pink, then flip them over and add the unsalted butter. Cook for an additional 2-3 minutes until the shrimp are fully cooked and the butter has melted, creating a rich sauce.
Season the shrimp with salt and pepper to taste. Remove from heat and garnish with sliced green onions before serving. Enjoy your Cajun Garlic Butter Shrimp over rice, pasta, or as a standalone dish.