Preheat oven to 425°F.
Clean mushrooms and pat dry.
Remove stems from mushrooms. Chop stems and set aside.
Add butter to small sauté pan over medium-high heat. When butter liquefies, brush the tops of the mushroom caps and place them topside down on a baking sheet lined with foil or parchment paper.
Sauté onions, chopped mushroom stems and garlic in remaining butter, until moisture is reduced and onions begin to brown. Let cool slightly.
Remove from heat and stir in Panko breadcrumbs and then shredded cheese.
Fill mushroom caps.
Bake 10 to 12 minutes.
To make ahead, follow steps 1-7 and refrigerate until you are ready to bake.