In a bowl, whisk together orange juice fish sauce, cornstarch and soy sauce. Stir in beef, tossing to coat. Marinate at room temperature for 30 minutes or refrigerate for up to 1 day.
In a large non-stick frying pan, heat oil over medium high heat. Cook onion and garlic for 5 minutes or until softened. Add curry paste and cook for 1 minute. Add beef and cook for 2 minutes or until browned abut still pink inside. Stir in coconut milk, peanut butter, lime juice, and Splenda. Cook, tossing to coat meat with sauce, for another 2 minutes. Garnish with cilantro and peanuts.