In a bowl, combine thinly sliced beef tenderloin or sirloin with oyster sauce, soy sauce, salt, and ground black pepper. Mix well to ensure the beef is evenly coated and allow it to marinate for at least 30 minutes.

Heat a couple of tablespoons of cooking oil in a large wok over medium-high heat until it shimmers.

Add minced garlic, freshly grated ginger, and sliced chili to the hot oil. Sauté for 1-2 minutes until fragrant and golden, stirring constantly.

Toss in the fresh spinach and stir-fry for 2-3 minutes until wilted and bright green. Transfer to a serving plate to keep warm.

Add the marinated beef to the hot wok in a single layer and sear for about 5-6 minutes until rich brown and cooked through.

Gently fold the beef over the spinach on the serving plate, drizzling any leftover sauce from the wok over the dish. Serve warm.