01 -
Set the oven to 200°C (400°F) to preheat.
02 -
Combine Mozzarella, Gruyère, Parmesan, parsley, Italian seasoning, onion powder, and garlic powder in a medium bowl; mix thoroughly and set aside.
03 -
Remove mushroom stems and slice portobello caps into 1 cm thick pieces.
04 -
In a 2-litre baking dish, arrange one-third of the mushroom slices evenly; sprinkle with one-third of the cheese mixture, then season with salt and cracked pepper. Repeat this process two more times to create three layers.
05 -
Mix heavy cream with minced garlic and pour evenly over the layered mushrooms.
06 -
Cover the dish with foil and bake for 30 minutes, until mushrooms are tender.
07 -
Remove foil, carefully pour the excess liquid from the baking dish into a small saucepan, and set aside.
08 -
Return the mushroom dish to the oven under broil/grill for 4–5 minutes until the top is golden brown.
09 -
Bring the reserved mushroom liquid to a simmer over medium-high heat.
10 -
Whisk together cornstarch with water into a slurry, then gradually stir into the simmering liquid. Continue stirring until the gravy thickens.
11 -
Plate the scalloped mushrooms and spoon the thickened gravy over the top. Garnish with additional chopped parsley before serving.