Begin by slicing your beef tenderloin or sirloin thinly against the grain for tenderness. Combine it in a mixing bowl with oyster sauce, soy sauce, salt, and ground black pepper. Let it marinate for at least 30 minutes.

In a large wok, heat 2 tablespoons of cooking oil over medium-high heat. Add minced garlic, ginger, and fresh chili, stirring constantly for about 1-2 minutes until fragrant.

Toss in the fresh spinach and stir-fry for 2-3 minutes until it wilts. Transfer the spinach to a serving plate.

In the same wok, add the marinated beef in a single layer. Cook for approximately 5-6 minutes, stirring frequently until browned and cooked through.

Carefully arrange the beef on the bed of wilted spinach. Serve hot alongside steamed rice or noodles.