Combine the beef, oil and garlic in a small bowl. Heat a wok or a heavy nonstick frying pan over high heat. Add the beef strips in small batches and stir-fry for 1 to 2 minutes. Remove each batch when cooked and allow the pan to reheat before frying the next batch.
Add the capsicum and mushrooms to the pan with a sprinkling of water. Steam for 1 to 2 minutes, then return the beef strips.
Stir in the curry paste, brown sugar, fish sauce, coconut milk, spinach and coriander. Toss for 1 to 2 minutes or until heated through. Serve with rice, if liked.