Place green beans in a Dutch oven; add water to cover. Bring to a boil. Cook, uncovered, 3-4 minutes or just until tender. Drain.
Meanwhile, in a large skillet, heat butter over medium-high heat. Add mushrooms and onion; cook and stir 3-4 minutes until tender. Add garlic; cook 1 minute longer.
In a small bowl, whisk flour and 2 tablespoons milk until smooth.
Whisk in remaining milk; stir into mushroom mixture. Bring to a boil, stirring constantly. Cook and stir for 2 minutes. Stir in dressing mix and pepper. Add green beans; heat through. Transfer to a serving bowl; sprinkle with bread crumbs.