Heat the butter in a large skillet over medium-high heat until melted and gently sizzling.
Add the sliced mushrooms and toss to coat evenly. Sauté for several minutes, stirring occasionally, until the mushrooms release their moisture.
Add the garlic and continue cooking until the liquid fully evaporates and the mushrooms turn golden brown and slightly caramelized, about 10–12 minutes. Adjust heat to avoid burning.
Drizzle in the tamari and stir well to glaze the mushrooms evenly. Taste and adjust seasoning.
Serve the garlic butter soy-glazed mushrooms immediately while hot.