Give the Scouts a list to choose from and then let them create their own main meal - mark them out of 40 (10 points each for Taste, Quantity, Presentation, Timing)
Food on the list, mixing bowls, pots, pans, utensils, cookers/stoves, cleaning/washing up equipment etc
Allow each Scout to pick one item from each of the following categories: Staples (e.g. potatoes, pasta, rice, cous cous, noodles etc); Protein (e.g. sausages, minced beef, diced chicken, tuna, butternut squash, mushrooms etc); Veg/Salad (e.g. peas, carrots, sweetcorn, lettuce/tomatoes/pepper/cucumber etc); Others (eg. packets of various powdered sauce mixes, condensed soup, pesto, passata/tinned tomatoes etc). Give them access to the "Store Cupboard" (i.e. salt, pepper, cooking oil, stock cubes, onions, garlic, herbs, chilli powder, worcestershire sauce, soy sauce, oyster sauce, ketchup, milk etc).
For Explorers/Adults, restrict the quantities of each ingredient (e.g. so they cannot all have pasta or all have tinned tomatoes) and get them to take it in turns to pick from each category so no one has first choice from every category.
You will need to allow the choices to be made in advance so you can get all of the required ingredients. For Scouts, they could do it in pairs or patrols.