In a large skillet, heat the vegetable oil over medium-high heat. Add the beef chunks and sear on all sides until browned. This should take about 5-7 minutes.
Transfer the browned beef to the slow cooker.
In the same skillet, add the diced onion, minced garlic, and grated ginger. Sauté until the onion becomes translucent, about 3-4 minutes.
Stir in the red curry paste and cook for an additional minute until fragrant.
Pour in the coconut milk, fish sauce, and brown sugar, mixing well to combine.
Pour the mixture over the beef in the slow cooker, ensuring the beef is well coated. Season with salt and pepper.
Cover and cook on low for 6-8 hours or on high for 4-5 hours until the beef is tender.
About 30 minutes before serving, add the sliced bell pepper and spinach leaves to the slow cooker.
Once cooked, stir in the lime juice for a fresh zing. Taste and adjust seasoning as necessary.
Serve hot, garnished with fresh cilantro over rice or with crusty bread to soak up the delicious curry sauce.
Notes
Serve in deep bowls with a sprinkle of cilantro on top and lime wedges on the side for an added burst of fresh flavor.