Highly reputed chef Monica Galetti and Michelin-starred Marcus Wareing are together once again in their search for the UK's most exciting cooking talent. Tonight's last quarter-final sees the six strongest chefs from this week's heats attempt to impress Monica and Marcus in two crucial challenges. Only the three most talented chefs will go through to knockout week and continue their pursuit of the MasterChef: The Professionals title. First up, the professionals face an invention test, in which the key ingredient specially selected by the judges must feature. As Monica has chosen squash this week, the chefs are given a larder with a wide range of pumpkins, squash and other sweet and savoury ingredients to choose from. They have 70 minutes to cook a dish that shows off their versatility and creative nous. This dish has to be worthy of seeing them through to the next vital round. Only four chefs will succeed and have the chance to cook for the restaurant critics. The four chefs that showed star quality now face a challenge that has sent even seasoned chefs into a spin — the critics test. They cook their own two-course menu to be scrutinized by the judges and three of the UK's most discerning palates. Food critics Jay Rayner, Jimi Famurewa and Tracey MacLeod are eager to serve up their opinions on the chefs' food, which can range from delight to disdain. After this round, one chef is shown the door while the other three survive to battle it out in knockout week.