Heat a large non-stick skillet over medium-high heat. Add beef; cook 8-10 minutes or until browned, stirring occasionally. Drain. Place beef in a 4-quart electric slow cooker; sprinkle with salt.
In a medium bowl, combine beef broth, sugar, curry paste, fish sauce, lime juice, coconut milk and sriracha; pour over beef. Cover and cook on LOW for 6-7 hours.
Stir in spinach. Whisk flour in 2 tablespoons water; stir into slow cooker. Cover and cook on LOW for 15 minutes or until sauce thickens.
Serve beef mixture over steamed rice; sprinkle with basil leaves.