Steamed Chinese cabbage in supreme soup is among the signature dishes of Zen restaurant in Beijing. [Photo provided to China Daily]

"There are other young chefs who would like to replicate the classic dishes from the 1960s and '70s, but they don't know if they can cook them properly," says Wu.

"We were lucky because when we wanted to learn about a classic dish, we would ask my father-in-law and discuss how to make the dish with him. The recipes in the book are a bit general, so we needed his knowledge to replicate them properly."

Most of the dishes at Zen are from Sichuan cuisine, including its signature dish-kung pao chicken. The couple's take on this classic sweet-and-sour dish is both spicy and viscous, but the chilis are more of a red-brown hue rather than the usual flaming red color.

"The chilis in the original kung pao chicken at the Beijing Hotel were cooked to almost a black color, which can be verified by a color photo that appears in one version of the recipe book," says Wei Yuanjun. "But they are edible and the aroma of the chili is more prominent in this dish."

The couple aim to help diners understand that spiciness is not the only aspect to Sichuan cuisine, which has more than 20 flavors, including sweet-and-sour, garlic and sesame.