Step 1: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Brown the beef pieces in batches until seared on all sides, then transfer them to your slow cooker.
Step 2: In the same skillet, add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and grated ginger, cooking for another minute until fragrant.
Step 3: Stir in the tomato paste and curry paste, cooking for 2 minutes to toast the spices and deepen their flavor.
Step 4: Pour in the crushed tomatoes, stirring to combine everything, and bring to a simmer for a few minutes.
Step 5: Transfer the tomato-curry mixture to the slow cooker with the beef. Add the halved baby potatoes, chopped red bell pepper, and full-fat coconut milk. Stir gently to combine all ingredients.
Step 6: Cover and cook on low for 6-8 hours, or on high for 3-4 hours, until the beef is incredibly tender and the potatoes are cooked through.
Step 7: During the last 15-20 minutes of cooking, stir in the baby spinach until it has wilted. Taste and adjust seasoning with salt and pepper if needed. Serve hot with rice or naan bread.