Saute the onions slowly on a low heat in the olive oil. For a long while. I think I sauteed mine for about a half an hour. The onions get really soft and a pretty brown color. Add the carrots and celery until those get soft. Then sprinkle the flour on top of the veggies. Mix well.
Add the beer and the chicken stock. Let it simmer. Then add the rest of the ingredients Simmer for at least 30 minutes.