Preheat the oven to 200°C (400°F), Gas mark 6. Place the garlic and cream in a small heavy-based pan over a very low heat and simmer gently for about 45 minutes until the garlic is completely tender and the cream has reduced and thickened, stirring occasionally to make sure the bottom of the pan doesn’t catch and burn.
Remove the garlic and cream mixture from the heat and mash to a rough purée with a fork. Season to taste and leave to cool, then stir in the panko or breadcrumbs and fold in the cheese and thyme.
Melt the butter in a small pan and use to brush all over the mushrooms, except the gills, as the filling will cover that. Arrange the mushrooms on a non-stick baking sheet, top-side down and fill the centres with spoonfuls of the garlic cream and cheese mixture. Bake for 15-20 minutes until the mushrooms are cooked through and the topping is golden brown.
To serve, transfer the garlic stuffed mushrooms with cheese to warmed plates and arrange a little rocket salad (Arugula) to the side of each one.