This shot bothered me initially. When professional chefs hold knives they hold the blade itself between their thumb and forefinger. It is supposed to give them more control, precision and speed. As an owner of several restaurants one of which has two Michelin stars, surely Mycroft would know better?
However, it occurred to me that it is possible that Mycroft was never professionally trained. Perhaps it is something that he dabbled at first. He just did it as a hobby as a teenager in order to distract himself from the monotony and oppressive air of boarding school.
Then when he reached his majority and got access to his trust funds, he was suddenly met with new found freedom and substantial money to really make use of said freedom. So he decided to open a restaurant. Why not? After all food was something that he was passionate about and gave him joy during his mostly joyless teenagehood.
And that sort of sums up my impression of Mycroft - someone with an air of nonchalance, someone who lives with certain carefree-ness and gives into his whims, someone who is more concerned with living each day than achieving some grand goal or purpose.
Which, in my opinion, is a stark contrast from Sherlock. So maybe that is why Mycroft come across as lazy and ambitionless to Sherlock. Also I feel as though Sherlock is on some level envious of Mycroft attitude towards life and the fact that Mycroft does not have to battle the overactive obsessiveness that he has to battle everyday.
But Mycroft is not a professional chef in the story. He opened and owns restaurants but he is not the chef.