Provide the basic ingredients for a burger(/pizza/etc) and ask the scouts to bring along extra ingredients to make it the best burger/pizza/etc for a competition
For burgers - mince, onions, eggs, buns etc
Pizzas may work although cooking them all may be tricky:
For pizzas - dough (or pizza bases), tomato sauce, cheese
Taken from Big Chris on escouts - https://www.escouts.org.uk/forum/threads/28849-Burger-and-Milkshake-Night
The scouts bring (from home) their magic ingredients to make the best burger ever, bringing enough to make two burgers.
They assemble & cook their burger and then as a patrol, decide who's creation is the best and then cook that again to be judged by the poor judges who will be force-fed burgers to rate them!
Could do a similar thing with drinks/milkshakes/puddings/etc for a two-course meal.