In a large pot or Dutch oven, heat the butter and olive oil over medium heat. Add the onions and a small pinch of salt and cook over medium-low heat for 15 minutes, stirring occasionally, until lightly golden brown. Add the garlic and cook for another minute. Sprinkle in the flour and stir, cooking for another 3 minutes.

Stir in the chicken stock and tomatoes. Bring the soup to a boil, then lower the heat and simmer for 15 minutes. Purée soup with an immersion blender (or in batches in a regular blender) until smooth. Stir in the sour cream and serve.