Add your olive oil to a large heavy pot and prep your garlic, onion and celery. Pre-heat the pot and sautee the veg, seasoned with sea salt, for 7 to 10 minutes, stirring periodically to ensure that garlic does not stick to the bottom of the pot and burn.
Add herbs and freshly ground black pepper and stir to combine. Saute for another minute or so until wonderfully fragrant. Add flour and stir to combine well and cook for another minute or so.
Add tomato paste and and stir to coat veg and cook for another minute.
Add all of the veg and stir to distribute aromatics throughtout and pour in the stout (yup, it's vegan . . . thank you Guiness!) Bring to a boil and let cook for a few minutes so that that veg can take on some of the stout. If you are not using the stout, add all of the mushroom or vegetable stock at once in the next step.
Add the veg stock and worcestershire sauce and bring to a boil before reducing to a simmer and covering for about 30 minutes, stirring every 10 minutes or so and replacing the cover.
The flour that was cooked with the aromatics should be enough to thicken your stew. However, if you find it could be a little thicker, make a slurry out of a teaspoonful each of flour and arrowroot starch (or cornstarch) and water. Bring the stew back to a boil and add just enough of the slurry to thicken to the desired consistency.