Preheat oven to 200°c-390°f.
In a mixing bowl, combine the mozzarella cheese, Parmigiano Reggiano, flat-leaf parsley, onion powder, dried oregano, dried thyme & dried basil. Mix together & set aside.
In another bowl, combine the thickened cream & garlic. Whisk together & set aside.
Remove the stems from the portobello mushrooms, that way they can sit flat. Slice the mushrooms & stems into 1cm strips. Lay the mushrooms & stems neatly in a baking dish. It's okay to stack them on top of one another.
Pour over the cream & garlic mix & sprinkle over the cheese & herb mix. spread it out evenly ensuring everything is covered.
Cover the baking dish with aluminium foil & bake for 25 minutes. After 25 minutes, remove the foil, return the mushrooms back to the oven and grill/broil for 5 minutes or until the cheese is golden brown. Dig in.