Heat oil over medium-high heat in a Dutch oven or large, high-sided saute pan. Add onions and cook until translucent, then push them to one side of the pan. Add beef and brown on all sides. Stir in carrots and celery and cook for 3 more min. (If using a smaller pan, cook onions, beef, and carrots/celery in separate batches, then combine.) Sprinkle with flour and cook until flour picks up all of the fat in the pan and starts to smell nutty, about 2 min. Add broth and Guinness to deglaze pan. Add potatoes, mushrooms, garlic, bay leaves and The Q Steak Rub. Bring to a boil then reduce heat and simmer for 20 min. to an hour, or until beef is tender and stew is to the consistency of your liking. If it has trouble thickening, add 1 Tbsp. tomato powder at a time (made into a paste by adding water) to help thicken the stew. Season with salt & pepper to taste before serving.