to prepare it at home it was smart move on his part, he is a clever one, he knows how to get rid of her but keep on doing what he finds reasonable, he knows he cannot communicate with her directly so this is how it is done, he handles her well
if he would cook it there she would easily have said well great you know the way around kitchen so here you go you will be cooking I have someone else for the garden, but he showed her that for his TRAINED level of cooking he needs professionally equiped kitchen (even if he had non at home) and cooking at home and bringining it to lalaland would not be the way she wants her cook to work, she wants them frustrated without decent workspace to work on, with people standing in a way and equipement nowhere to find, malfunctioning oven included, she wants to witch and order them around and show them who is the boss with passive aggressive smirk, and one better be on time because you know the guests are waiting her majesty