Combine the minced garlic, olive oil and balsamic in a small bowl. Divide the Portobello mushrooms between two gallon-sized resealable bags (two per bag) and pour half of the marinade into each. Toss to coat the mushrooms in the marinade and then place in the refrigerator for at least 30 minutes, or up to a couple of hours.
Once mushrooms are done marinating, preheat the oven to 350 degrees Fahrenheit and begin to make the stuffing. Melt the butter in a medium sauté pan over medium-high heat. Once the butter has melted most of the way, add the sage leaves. Let the butter finish melting and continue to cook until brown specks begin to appear. Remove the sage leaves and reserve for later.
Add the breadcrumbs and panko to the butter on the stove. Chop up the reserved sage leaves and add those back in as well. Cook over medium heat until the breadcrumb mixture begins to turn light golden brown. Pour into a large mixing bowl and set aside.
Spray the same sauté pan with some nonstick cooking spray and then add the bacon. Cook until golden and just crispy. It is important not to overcook the bacon in this step. Once cooked, remove from pan and set on a plate lined with a paper towel to drain.
Next, in the same sauté pan and using the grease from the bacon, add the sausage. Cook until lightly browned, then add to the mixing bowl with the breadcrumbs. Chop up the bacon and add that as well. Finally, add the cheese.
Remove the mushrooms from their plastic bags and place on a baking sheet lined with aluminum foil. Pour any remaining marinade into the stuffing mixture and stir to coat. Divide the stuffing up amongst the four Portobellos and place in the oven. Bake for 15 minutes.
Remove from oven and let cool slightly before serving. Perfect when paired with a light green salad.