Cook green beans in saucepan of salted boiling water, 2 to 3 min. or until tender-crisp. Drain. Transfer to foil pan. Toss with 2 tbsp (30 mL) melted butter, mustard, garlic and green onions. Top with Cheddar cheese. In bowl, toss breadcrumbs with remaining 2 tbsp (30 mL) melted butter and Parmesan cheese; sprinkle on top of Cheddar. Cover pan with foil. (Note: Recipe can be made to this point 1 day in advance, covered and kept chilled. Bring to room temperature before proceeding to step 2.)
Place covered foil pan on grill preheated to medium. Cook with lid closed 6 min. Remove foil and cook another 6 to 7 min., or until beans are tender and cheese is melted. Serve warm or at room temperature.