In a wok, heat one tablespoon oil on high, toss in beef slices, and minced garlic. Stir until the beef browns lightly. Set aside. Heat the remaining oil in wok, add the onion and pepper, followed with mushrooms, and stir-fry until all the vegetables are tender. Add curry paste, coconut milk, and fish sauce, bring to a boil, stirring occasionally. Reduce heat, simmer, and add beef and spinach. Stir often until the beef is cooked.