In a large bowl, combine beef strips and cornstarch; toss well.
Heat the oil in a skillet with a tight-fitting lid over medium-high heat; add steak strips and sauté for 5 minutes; add onion and sauté for 1 minute. Add mushrooms.
Cover and cook for 2 minutes then add broth, salt and pepper. Reduce heat and simmer, uncovered, for 5 minutes. Remove from heat; stir in sour cream* and chopped parsley.
Sprinkle paprika over the top; serve over chopped spinach.
*LEANNE’S NOTE: Re-heating at a high heat will cause the sauce to break, because of the sour cream.