What to call a small restaurant which serves only a single type of food? [duplicate]

I don't think this precisely hits your definition, but a hole-in-the-wall restaurant refers to the size and inconspicuousness of a restaurant.

That's not the same as having a limited menu, but hole-in-the-wall restaurants are often exactly those that have a strong focus on their specialty menus.

You could describe the place as a boutique restaurant.

Merriam-Webster's second definition of boutique is:

a small company that offers highly specialized services or products

boutique wineries

an independent investment boutique

This word has both the connotation of smallness, as well as the highly specialized product which the single type of food would represent.