Heat a large skillet over medium-high heat. Add beef; cook and stir until browned, about 2 minutes per side. Drain excess grease. Transfer beef to a 4-quart slow cooker and sprinkle with salt.

Cook onions and garlic in the same skillet over medium-high heat until tender, about 5 minutes. Add to the slow cooker.

Stir coconut milk, beef broth, red curry paste, jalapeños, lime juice, peanut oil, and brown sugar into the slow cooker until well combined.

Cover slow cooker. Cook on Low until flavors combine, 6 to 10 hours.

Bring water and rice to a boil in a large saucepan. Reduce heat to medium-low, cover, and simmer until rice is tender and water is absorbed, 20 to 25 minutes.

Meanwhile, stir spinach into the slow cooker; cook until wilted, about 15 minutes.

Serve beef mixture over rice; garnish with basil leaves.