The Dutch information office of nutrition published a recipe for soup made of Dahlia tubers in the winter of 1944-1945: 1 1/4 litres of water, 1 kilogram of Dahlia tubers, 1/2 kilogram onions, salt, and a surrogate for pepper. The Dahlia tubers were to be washed and then rasped or the be cut in small pieces. The onions were to be cut in similar small pieces and to be baked alongside the Dahlia tubers in a little bit of oil. After adding the water and the salt, the whole thing had to cook for 30 to 45 minutes. It had to be finished off with a little of the pepper surrogate. That tulip bulbs were eaten in the Netherlands during the Second World War is common knowledge, but that the same thing was true for Dahlias has been long forgotten. That is mostly because tulips were much wider available and have therefore saved more lives; still, Dahlia's deserve a bit of the credit, too. We were however not the first to eat Dahlia's: the Aztecs who used to live in the land that is now Mexico, where the Dahlia originates from, used to eat Dahlia tubers as well.
Oh, right, something about Penhill Dark Monarch in particular: like her sister Penhill Dark Watermelon, she is a Dahlia with cheerful flowers. They go well together, so combining them in your garden might be a recommendation.