Q. Is this use of the passive voice correct? “The restaurant’s excellent dinners had been being prepared by Chef Bob for many years.”
A. Not quite. Passive voice can be a good way to emphasize the results (excellent dinners) rather than the action that produced them (Chef Bob’s preparation). But as the grammatically redundant “had been being” reveals, there’s no such thing as a passive form of the past-perfect progressive tense—that is, the verb tense that describes an ongoing action that occurred in the past but ended at some definite point (also in the past), whether specified or implied.
To correct the grammar of your example, you’d have to switch to the past-progressive tense:
The restaurant’s excellent dinners were being prepared by Chef Bob for many years.
Or, as the better option, you could use the past-perfect tense alone:
The restaurant’s excellent dinners had been prepared by Chef Bob for many years.
The past-perfect may be the better option, but neither of those choices captures the sense of the past-perfect progressive. To use that, you’ll have to switch to the active voice:
Chef Bob had been preparing the restaurant’s excellent dinners for many years.
For a review of progressive tenses, see CMOS 5.135.