In a medium bowl, whisk together the low sodium soy sauce, light brown sugar, whole grain mustard, and Cajun seasoning.
Add the peeled and deveined shrimp to the marinade, stirring to coat them evenly. Let them sit for about 10-15 minutes.
In a large skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 30 seconds.
Add the marinated shrimp to the skillet in a single layer. Cook for about 2-3 minutes until they begin to turn pink, then flip them over.
Once the shrimp are flipped, add the unsalted butter to the skillet. Allow it to melt and mix with the shrimp, creating a rich sauce. Season with salt and pepper to taste.
Remove from heat and garnish with sliced green onions. Serve over rice, pasta, or on its own.