Cut beef Top Sirloin Steak lengthwise in half, then crosswise into 1/8-inch thick strips. Combine garlic and thyme in large bowl. Add beef; toss to coat.
Heat 1 teaspoon oil in large nonstick skillet over medium-high heat until hot. Add half of beef; stir fry 1 to 2 minutes or until outside surface is no longer pink. Remove from skillet. Repeat with remaining beef. Season with 1/4 teaspoon salt and pepper. Keep warm.
Heat remaining 1 teaspoon oil in same skillet on medium-high heat until hot. Add onions and mushrooms; cook 8 to 10 minutes until onions begin to caramelize and mushrooms are tender and begin to brown, stirring occasionally. Sprinkle flour over onion mixture; cook and stir 2 minutes. Stir in broth and remaining 1/4 teaspoon salt. Bring to a boil. Cook and stir 4 minutes until thickened. Return beef to skillet; cook on medium-low heat 2 to 4 minutes until heated through. Remove from heat; stir in 1/4 cup sour cream.
Season with salt and pepper, as desired. Serve over noodles. Top each serving with a dollop of remaining 1/4 cup sour cream. Garnish with parsley, if desired.
Nutrition information per serving: 408 Calories; 126 kcal Calories from fat; 14 g Total Fat; 6 g Saturated Fat; 4 g Monounsaturated Fat; 122 mg Cholesterol; 575 mg Sodium; 32 g Total Carbohydrate; 2.7 g Dietary Fiber; 35 g Protein; 3.4 mg Iron; 9.9 mg Niacin; 0.7 mg Vitamin B6; 1.5 mcg Vitamin B12; 5.3 mg Zinc; 38 mcg Selenium; 112.7 mg Choline;