Called Hector's, the concept is inspired by the cave à mangers of Paris and drinking holes of San Sebastian.
Stephenson, who over the last seven years has built the wine list at Hill & Szrok Butchers and Cookshop up from six to 300 bins, will sell a mixture of natural and classic European wines through the bottle shop, which has already launched.
The bar, meanwhile, which opens in July, will have an emphasis on dynamic wines and small plates at accessible prices.
A cellar list of 'unusual and rare' vintages will also be available, and guests will be able to drink-in any of the bottles on sale at the bottle shop for a corkage fee of £15.
Luca Mathiszig-Lee, owner of Hill & Szrok, is on board as a partner at Hector’s, and Stephenson will run the business with his wife, actress Anna Shaffer.
The food offering will be a selection of small plates that draw on Stephenson and Shaffer’s travels and love of European bar food, and an emphasis on local suppliers.
Dishes set to feature include Cantabrian anchovies soaked in olive oil with rose petals; courgette carpaccio with fresh cheese; salted Catalan almonds; and pâté en croûte made especially for Hector’s by butcher George Jephson.
There will be a 'concise' menu of rotating bocadillos available from lunchtime through until the evening, such as tortilla with Spanish red peppers; and jamón con tomate, with bread supplied by Jumi in Newington Green.
Guest chefs will also be invited to collaborate for takeover weekends and events.
"I’m so excited to be bringing a slice of Europe to De Beauvoir," says Stephenson.
"Not only do I love the relaxed but enthusiastic European way of eating and drinking but the cultural emphasis on family.
"This is our family business and it’s all about unfussy but tasty food and classic and exciting wines.
"I want to create a spot I would want to buy wine from but also a place I would feel welcome to pull up a chair and settle down for an evening. I hope people will feel at home here.”
Stephenson has designed a central table with a dual purpose of day time retail counter and evening bar, which will be surrounded by banquette seating and counters.