Thaw frozen vegetables in the microwave (or place entire bag in a bowl of hot water for about 10 minutes).  Set aside until step 4.

Combine all ingredients for broth, except soy sauce, in a medium-sized pot or saucepan.  Bring to a boil over high heat, then lower heat and simmer for 15 minutes.

Strain the broth through a fine wire colander, and discard the solid parts.  Season to taste with soy sauce.

To finish the soup, bring the broth back to a boil.  Add the thawed vegetable stir-fry mix and mushroom caps, and simmer for 1 minute.

Add the noodles and continue to simmer for another minute.

Add the beef and continue to simmer for about 5 minutes or until the beef is slightly pink to brown (to a minimum internal temperature of 145 ºF).

Add tofu and scallions, and simmer 1–2 minutes until heated through.

Serve immediately in 1-cup portions.

Tip:There are several varieties of tofu, each with a different moisture level.   Silken and soft tofu are the moistest and easily blended into shakes, dips, and dressings.  Regular tofu is less moist, and it's best for scrambling or using like cheese in casseroles. Firm, extra-firm, and pressed tofus are the driest. They absorb other flavors easily and hold their shape in stir-fries and on the grill.

source: National Heart Lung and Blood Institute