I'm designing a creature that is supposed to be able to eat "basically anything", within the realm of plausibility for what is essentially an organic creature. Its digestive system can be as complicated as necessary, and evolutionary history is not an obstacle, the creature could be either alien or engineered but one way or another it is omnivorous to the extreme.
At the very least, it should be able to break down any organic, energy-storing molecule, ranging from plants and meat to fossil fuels. Any other naturally-occurring chemical with a high energy content should be edible as well, if possible.
Notably though, it is an active, mobile animal so it should only be interested in eating things that have a relatively high energy content. It is possible to get energy by oxidizing metal but probably not enough to sustain a high-energy lifestyle.
How would the creature's digestive system work, in order to give it the ability to digest as many different things as possible?