Tonight's quarter-final sees the strongest four chefs from heats three and four return to face two more demanding challenges.First, the judges set the bar high with an Invention Test. The chefs must create a standout dish centered around one key ingredient, mustard. They have ten minutes to select their ingredients from an array of meat, fish and vegetables, then 60 minutes to cook and serve the dish. Can they turn up the heat and balance the bold and complex flavours of the classic condiment?Then they face the daunting task of cooking for three of the UK's leading restaurant critics: Grace Dent, Tom Parker Bowles and William Sitwell. With one opportunity to make their mark, they have one hour and 15 minutes to prepare a perfect two-course menu. Can they strike the right balance between ambition and capability?Only the most talented chefs will make it through to the semi-finals.