Rub enough olive oil onto cod fillets to coat; season with salt and pepper.
Heat about 1 tablespoon olive oil in a skillet over medium-high heat; saute onion, red bell pepper, and garlic until tender, about 2 minutes. Stir diced tomatoes into onion mixture. Arrange cod fillets on top of tomato mixture and pour coconut milk over fillets. Sprinkle green onion over cod fillets and top with Sriracha hot sauce.
Cover skillet, turn heat to low, and cook until fish flakes easily with a fork and stew is thickened, about 1 hour.