Gregg Wallace is in Cornwall at an enormous bakery where they produce 180,000 Cornish pasties a day. He follows the production of the pastry snacks from the arrival of two tonnes of swedes right through to dispatch. Gregg learns that there are very specific rules to creating a Cornish pasty. They must be made in Cornwall, the filling can only contain onion, potato, swede, beef and some seasoning - and each ingredient must be cooked from raw within the pastry parcel.Meanwhile, Cherry Healey is delving into the wonderful world of the onion. She peels back the layers to discover the science that makes it such a versatile vegetable, and more importantly, why it makes us cry. It is all down to a chemical called lachrymatory factor, which is only created when an onion is cut into. Cherry visits an anaerobic digestion plant, where they turn waste from food factories into electricity. Micro-organisms feed on food waste, producing methane gas, which is used to power generators.Historian Ruth Goodman is debunking some common Cornish pasty myths. It has been claimed that the county's tin miners invented the pasty as a convenient snack to eat while they toiled at the rock face. She learns that miners may have eaten them, but they didn't invent them. And it is unlikely that they used the pastry crimp as a handle. She also visits the Worshipful Company of Grocers in London, which was responsible for importing pepper into Britain. She learns how this ubiquitous seasoning transformed from a commodity so valuable it was known as black gold to a spice that everyone could afford.