Heat butter in a Dutch oven to medium-high on the stove top. Add onions and celery and cook, stirring every few minutes, until the onions are soft and fragrant. Add flour, salt and pepper. Stir to coat the veggies with the flour and cook for one minute. Slowly whisk in the stock until all stock is used. Remove from heat and using an immersion blender, puree the soup until smooth. Stir in the cream and serve immediately.