I suggest making your restaurant an ice cream parlor.
Most modern food preparation techniques were used in the middle ages. They did not have microwaves, but steaming or putting something to stand near the fire could get similar results, just slower. It was certainly possible to create foods similar to modern ones if the ingredients were available. To see the variety of dishes that were actually prepared I suggest reading some medieval cook books.
The big difference is that the ingredients for modern cooking were either not available, or too expensive for most people to eat them. The people who could afford the quantities of protein (especially beef), refined white flour, spices, and sugar would eat at the high table in their own hall, not at an inn or tavern. A medieval celebrity chef worked for a VIP, rather than opening restaurants.
A few wealthy people in the right areas might be able to have ice brought from a mountain glacier, but most people in most areas had no access to frozen dishes if it was warm enough to make them enjoyable.
Fermented fruit juices, milk, eggs, and honey were all reasonably available at prices a merchant could afford, at least in small quantities as a treat. If you did your buying quickly, at the right time, you might even be able to get unfermented fruit juice into your freezer. It is sweeter, and freezes better. Cook everything to kill bacteria before freezing.
Based on some comments on another answer, the ice cream parlor could also offer, based on distillation, completely clear, safe drinking water with ice in it.