Preheat the oven to 200°C (180°C) fan-forced. Heat the olive oil in a medium frypan over high heat. Add the mushrooms and thyme and season well. Sauté for 5 mins, or until the mushrooms are golden brown.
Arrange 6 of the croissants onto the bottom of a medium-sized baking dish. Top the croissants with the mushroom mixture. Arrange the remaining croissants over the top and add chopped brie and weave through torn pieces of ham.
In a large jug, mix the garlic cream, milk, eggs and mustard and carefully pour it over the torn croissants. Leave to soak for 10 mins.
Cover the baking dish with foil, and bake for 25 mins, until the cream mixture starts to set. Remove the foil and bake for a further 5 mins, until the croissants are golden and the brie is melted. Serve warm.