Bring beef, stock and Seasoning to boil in large saucepan. Reduce heat to low; cover and simmer 1 1/2 hours. Remove beef and cut into 1/2-inch cubes. Reserve stock and skim off excess fat.
Heat oil in large skillet on medium-high heat. Add onion and bell pepper; cook and stir 4 minutes or until tender. Add cooked beef; cook and stir 2 to 3 minutes. Sprinkle with flour; cook and stir 2 to 3 minutes longer. Gradually add 1 1/2 cups of the reserved stock (including the herbed residue), stirring until thickened. Reduce heat to low. Simmer, uncovered, 15 minutes to blend flavors.
Serve in warmed flour tortillas with sour cream, chopped avocado, shredded cheese and chopped fresh cilantro, if desired.