Heat the vegetable oil in a large pot over medium heat. Add the onion and cook for 3 minutes until it softens.
Stir in the garlic and ginger, then cook for 1 minute until fragrant.
Add the Thai red curry paste and stir for 30 seconds to toast it lightly.
Pour in the coconut milk and vegetable broth. Bring the mixture to a gentle simmer.
Add the red bell pepper and cook for 3 minutes.
Place the tilapia pieces into the simmering broth. Cook for 4 to 5 minutes until the fish turns opaque and flakes easily.
Remove the pot from heat. Stir in the fish sauce and lime juice.
Scatter the Thai basil leaves over the stew just before serving.