I think the rules more or less already cover the result of the food production... someone with a high skill produces a good result.
Therefore,
anybody can attempt cooking, but only those really
well-trained get very good at it...like chefs.
I'd also disagree that the quality of food produced by Chefs is markedly any
better than that produced by someone at home(or in the field). The differences between the two lie in the presentation and the ingredients...and that is why there is a marked difference in cost.
Supermarkets in the UK basically have three ranges of food, Low Cost, Normal and Premium. Esentially the products are the same, with the Low Cost being poorer quality and no frills, with the Premium being slightly better quality than normal. The difference however between Normal and Premium is marginal at best with the majority of the difference being packaging/and or branding.
The same analogy can be used for the skill system.
Anybody can cook, even with no skill. -30 (i.e. no ranks).
Trained, even 1 rank basic, brings the skill result to +5.
A chef, therefore, simply has a better skill bonus, multiple specialisations and access to better ingredients (and also access to staff to help prepare it).
Where I would have the distinction (and therefore specialisation) is where recipies and techniques used are markedly different.
Take the examples of a fried egg, pancake or omlette and crepes suzette....the three are quite similar in cooking techiques/ingredients. I would grade the various recipies by difficulty of preperation.
Fried egg. = Routine.
Pancake = Easy.
Omlette = Normal.
Crepes Suzzette = Hard.
Basically, the specialisation element of what I was suggesting did make the skill a restricted skill for all other non-specialised aspects

. The "jump" in skill bonus when a trained cook acquires a specialisation, reflects the learning of specific techniques used by that specialisation.