In the bottom of a heavy duty pot, melt the butter. Add in the sliced mushrooms and cook until golden brown and caramelized. Remove and set aside.
Add two more tablespoons of butter to the pan and melt. Toss the pieces of steak in the flour, then add them to the pot. Save the rest of the flour for later. Brown the steak pieces until a nice, deep golden brown crust has formed. Add more butter to the pan as needed.
Stir in the carrots, celery, onion, garlic, bay leaves, and thyme and cook until the onions have sweat. About 3-4 minutes. Stir in the tomato paste and the rest of the flour until even distributed.
Pour in the ale, beef stock and half the balsamic vinegar and bring to a boil. Reduce to a simmer for 20-30 minutes or until the vegetables are tender. Strain out the bay leaves and thyme stems and add the second half of the balsamic vinegar and the mushrooms. Season with salt and pepper and serve with puff pastry croutons.
Notes
Take your time searing the steak and get a nice beautiful brown crust to each piece. This helps develop a stronger flavor.
This steak soup uses rustic vegetable cuts to keep with being a humble pub food. You don't have to be precise with them, but make sure that they aren't so large that they don't cook in time. Try not to have any larger than 1-inch pieces.
Use whatever your favorite ale is for this recipe, but I would avoid bright and hoppy beers like IPA's, hefeweizen, or lagers.
Waiting until the end to add the mushrooms prevents them from bloating with the soup broth. Instead, they become a garnish and preserve the golden-brown caramelization you worked so hard for.
For easy straining, put the herbs into a tea ball strainer that is easily removed from the soup. That way you don't have to fish around for them.