"The Slaves Of Spiegel" by Daniel Pinkwater.

This book is about a man named Steve Nickelson and a boy named Norman Bleistift who were kidnapped by junk food craving aliens from the planet Spiegel and forced to cook for them. Steve and Norman both worked as cooks and managed a restaurant called the Magic Moscow in Hoboken, New Jersey. The Magic Moscow was an extraordinary restaurant because it featured ice cream sundaes which contained pizza dough, lobster, vegetables, and other peculiar ingredients. …

…and Norman became excited about it and wanted to win. They tried really hard to create the most delicious dish, but they only won second place. However, they were very happy in the end, because it turned out that the first place winners had to stay on Spiegel as slaves and cook for the aliens from then on. Steve and Norman figured out that it's sometimes better not to get what you think you want.