Was the weapon just a roll of foil? YES! And only the external layers were put into the oven? Yes!
Was the foil put in the oven in order to cook the blood? No, but I think I can call the
Gustavo and the murderer (I'll call him Kyle, since we never named him) were two of many chefs cooking at a reception at an apartment. Gustavo went onto an icy balcony to smoke; Kyle came up behind him and hit him with a roll of aluminum foil, killing him. Kyle wiped some of the blood onto the balcony railing (which was round and metal) to make it look like Gustavo had slipped on the ice and hit his head, then returned to the kitchen. He took the bloody outer layer of foil onto a pan, put another layer of foil on top to hide the blood, then put some food on the pan and put the pan into the oven he was using that night, intending to hide the evidence by throwing out the used foil. The body was found by another chef a few minutes later; the police were summoned and at first glance thought it was an accident.
However, when Detective Dexter examined the body, he noticed that the head wound was vertical, not horizontal as it would have been if Gustavo had hit the railing. Since that balcony was only accessible from the kitchen, Dexter deduced the weapon was another metal cylindrical object - a roll of foil - and opened all the ovens to look for foil that was different from the rest. Spotting the double layer of foil in Kyle's oven, he removed the pan and discovered the blood. Since only Kyle had used that oven, he was fingered as the culprit.
Well done everyone, and nicely done Acridian for that last key detail!