A friend told me that she has never been able to make a decent beef and vegetable soup, so I set out to find a recipe and make it. I found the basis for this soup on Cooks Illustrated. I made some changes I deemed necessary, made the soup, liked it, and gave some to my friend and a friend of hers. They loved it.
Combine beef and soy sauce in medium bowl; set aside for 15 minutes
Heat oil in large Dutch oven or pot over medium high heat until just smoking. Add mushrooms and onion; cook, stirring frequently, until onion pieces are brown and dark bits form on pan bottom, 8 to 12 minutes. Transfer vegetables to bowl.
Add beef and cook, stirring occasionally, until liquid evaporates and meat starts to brown, 6 to 10 minutes. Add tomato paste and garlic; cook, stirring constantly, until aromatic, about 30 seconds. Add red wine, scraping bottom of pot with wooden spoon to loosen browned bits, and cook until syrupy, 1 to 2 minutes.
Add beef broth, chicken broth, frozen mixed vegetables, bay leaf, thyme, and browned mushrooms and onion; bring to boil. Reduce heat and simmer for 15 minutes with the pot uncovered to evaporate some water and concentrate the flavors.
Add the potatoes, cover the pot, and simmer for 15 more minutes.
When the soup is finished, turn off the heat. Adjust seasonings with salt and pepper, remove the bay leaf and serve