1. Toss the beef, 1 tbsp of the oil, and the garlic together. Heat a large wok over high heat. In batches, add the beef and stir-fry for 30 seconds to 1 minute, until the beef starts to change color. Using a slotted spoon, transfer to a plate.

2. When all the beef has been stir-fried, add the remaining oil to the wok and heat until shimmering. Add the onion and pepper and stir-fry for 2 minutes. Add the mushrooms and stir for 2 minutes more, or until all the vegetables are tender.

3. Stir in the curry paste. Add the coconut milk, fish sauce, and sugar and bring to a boil, stirring. Reduce the heat to medium and return the beef to the pan, along with the spinach and basil. Cook, stirring often, for 3 minutes, or until the beef is heated through and the spinach has wilted.