French Dip Grilled Cheese

Name: French Dip Grilled Cheese
Ingredients: 8 slices white bread; 1/4 c. unsalted butter, melted; 16 c. slices provolone cheese (about 10 oz.); 1 c. slices thinly sliced deli roast beef (about 12 oz.); 1 c. French's Fried Onions; 4 tbsp. French’s Classic Yellow Mustard; 1 tbsp. unsalted butter; 1 clove garlic, minced; 1/2 tsp. dried thyme leaves; 1 1/2 c. low-sodium beef broth; 1 tbsp. French’s Classic Yellow Mustard; 1 tbsp. Worcestershire sauce; Kosher salt; Freshly ground black pepper; 1 tbsp. freshly chopped chives