Place cleaned mushrooms top side down on a large sheet pan. Set aside. Preheat oven to 350 degrees.
Skim or separate the fat from the gravy and pour the gravy into a medium sauce pan over medium heat. Simmer until the gravy has reduced by half. Add in butter and stir until well-combined.
Add in roast beef and cook until warmed throughout.
Using tongs or a large spoon, fill cavity of each mushroom cap with beef. Drizzle with remaining balsamic glaze.
Place in oven for 15-20 minutes until the mushroom has softened.
Serve warm.
Notes
Alternately to make this a make-ahead recipe, you can prepare recipe through adding the roast beef to the reduced gravy and then quickly assemble the stuffed mushroom caps. Cover tightly and store in the refrigerator until about 15 minutes prior to cooking.
15 minutes prior to cooking, as the oven is preheating to 350 degrees or you are heating the grill, remove the sheet pan of mushrooms from the refrigerator and allow to reach room temperature. Then, cook for about 20 minutes in the oven or on the grill. Serve warm.