In a small bowl, whisk together 1/4 cup low sodium soy sauce, 2 tablespoons light brown sugar, 2 tablespoons olive oil, 1 tablespoon whole grain mustard, 1 teaspoon Cajun seasoning, and 2 minced garlic cloves until combined.
Heat a medium skillet over medium-high heat and add 2 tablespoons unsalted butter. Heat until the butter is melted and bubbling.
Pour the whisked sauce into the skillet, whisk to combine with the butter, and bring to a simmer. Cook, stirring or whisking occasionally, about 1–2 minutes until slightly reduced and fragrant.
Meanwhile, pat 1 pound large peeled and deveined shrimp dry with paper towels and season both sides lightly with salt and pepper.
Add the shrimp to the skillet in a single layer, toss to coat in the sauce, and cook 4–5 minutes, stirring or turning once, until the shrimp are opaque and cooked through.
Remove from heat, spoon the sauce over the shrimp, and top with sliced green onions for garnish. Serve immediately.