Answer: Method of constant stimuli
Explanation: The chef is employing psychophysical method of constant stimuli.
Constant stimuli is one of the three psychophysical method, it depicts the sensory threshold by randomly presenting varying degrees of stimuli believed to have close proximity to the desired threshold.
Thus, in the chef scenario, her desired threshold was a subtle hint of black pepper flavor in a soup. Then, she continue to increase the amount of black pepper (i.e random presentation of stimuli) in her batches of soup, until she established a close proximity to her desired threshold (a subtle hint of black pepper flavor in a soup).