Place beef strips in a large bowl. In a small bowl, combine sherry (or juice/vinegar), soy sauce, gingerroot, garlic, sesame oil and cornstarch; blend well and pour over sirloin strips; stir to coat.
Heat a large skillet or wok over medium-high heat and coat with cooking spray. Add mushrooms, snow peas, zucchini and bell pepper; stir-fry until snow peas are bright green, mushrooms have wilted, and remaining veggies are tender-crisp. Remove from skillet and set aside.
Drain marinade from beef and reserve. Coat skillet or wok with cooking spray again as needed; stir-fry beef (half at a time) for 1 to 2 minutes. Return vegetables, beef and marinade to skillet; cook and stir until sauce thickens. Divide spinach evenly on each dinner plate; serve beef mixture on top.