Marinate the Beef: Place the petite tender beef in a resealable bag. Add the marinade sauce and minced garlic. Add the mushrooms into the bag as well. Seal and let marinate for 15–20 minutes.
Preheat the Grill: Heat a grill to medium-high heat until fully hot.
Grill the Beef and Mushrooms: Remove the beef and mushrooms from the marinade. Place them on the grill. Cook the beef for 4–5 minutes per side until nicely charred and cooked to about medium. Grill the mushrooms alongside until tender and slightly charred.
Prepare the Corn: Wrap each ear of corn in aluminum foil, sealing tightly.
Grill the Corn: Place the wrapped corn on the grill. Cook for 10–15 minutes, turning occasionally, until tender.
Make the Creamy Salsa Mixture: In a bowl, combine cilantro, sour cream, salsa, lime juice, salt, and black pepper. Mix until smooth and well blended.
Coat the Corn: Remove corn from the foil. Generously spread the creamy salsa mixture over each ear, coating evenly.
Add Cheese: Sprinkle crumbled cotija cheese over the coated corn so it sticks to the surface.
Rest and Slice the Beef: Let the beef rest for 5 minutes, then slice into medallions.
Assemble and Serve: Serve the sliced beef with grilled mushrooms and the dressed corn. Enjoy immediately while hot.