Remove stems from mushrooms; chop stems. Set mushroom caps aside.
Melt butter in a large skillet over medium-high heat; add shallots and garlic, and sauté 2 to 3 minutes. Add mushroom stems, and sauté 4 minutes or until tender. Remove from heat; stir in roast beef and next 4 ingredients.
Bring broth to a boil in a Dutch oven; reduce heat. Add mushroom caps in batches, and simmer each batch 3 minutes. Drain well. Spoon beef mixture evenly into caps. Sprinkle with breadcrumbs, and place on a lightly greased rack in a broiler pan.
Broil stuffed mushrooms 5 1/2 inches from heat for 3 minutes. Garnish, if desired.