It's quarter-final day. Two Michelin-starred chef Marcus Wareing, the formidable Monica Galetti and MasterChef's seasoned diner Gregg Wallace push the professional chefs to the limits as they hunt for Britain's next culinary superstar. The chefs' first challenge is the Invention Test - this one will push them to their limits. Whilst the chefs are used to cooking with the finest ingredients, they are instead faced with the kitchen scraps and trimmings, from which our judges expect them to create one course that truly impresses.