"And the onions, they've almost souffled in the centre." He's making a verb out of the noun souffle. He means that the onions have become light and fluffy, like a souffle. http://www.thefreedictionary.com/souffl%C3%A9

Occasionally you will hear people (at least in English, no idea about other languages) turn a noun into a verb, meaning to become either like the noun, or to describe a state of having something to do with the noun. It's often improper, but it happens (I myself am guilty of this on occasion). There are some instances as well where this has become correct (to brown a piece of bread for instance, means to cook it until the surface turns brown). Another example would be my "Before I start work in the morning, I have to get coffee'd up" = "Before I start work, I drink a lot of coffee"

It sounds like soufflé, but I've never heard it used as a verb before. It's probably cooking jargon.