Easy Beef Fajitas

• 4 flour tortillas
• 2 serrano
• 1 sirlon tip fat trimmed
• 1 cilantro (minced)
• 1/2 bottled salsa
• 240 ml nonfat yogurt (plain)
• 120 gr onions (sliced)
• 85 gr mushrooms (sliced)
• 80 ml dry sherry
• 5 ml safflower oil