Preheat the oven to 400 degrees. Line a baking sheet with parchment paper.
Rinse green beans and roughly chop them, then place in a mixing bowl. Roughly chop the half Brie wedge and place in the bowl along with the minced shallot. Add half of the fried onions, milk, salt and pepper. Stir well to combine.
Fill the mushroom caps with the mixture, topping them off with the remainder of fried onions. Place the caps on the sheet tray.
Bake in the oven for 20 to 25 minutes until the mushrooms are softened, the cheese is melted and the fried onions are a deeper, golden & crispy brown. Remove from the oven and serve.