Preheat oven to 300°F. Pour oil into a large wide saucepan or dutch oven and add onion. Set over medium-high and cook, shaking pan often, until onion is golden brown. Transfer onion to paper towels or clean tea towel to drain; season with 1/2 tsp. salt.

Return pan with oil to medium-high heat and add ginger and curry paste. Cook, stirring occasionally, until paste begins to stick to pan, about 3 minutes. Add tomatoes and cream and bring to a simmer. Cook, stirring occasionally, until tomatoes begin to burst and curry just starts to stick to pan. I followed the original directions and found that I could have simmered it much longer to get the tomatoes more broken down and the sauce a little thicker. Follow your preferences for a soupier or thicker curry at this point, because the oven stage won't thicken it too too much. Remove from heat.

Season fish fillets/tofu all over with 1 tsp. salt and arrange over tomato mixture. (if you are using chicken, I might suggest transferring some of the sauce into a separate baking tray or vessel and starting them baking sooner than the tomatoes because I think chicken might require longer bake times) Transfer pan to oven and bake until flesh is opaque all the way through, firm to the touch, and easily flakes when thickest part is pierced with a paring knife, 23-30 minutes, depending on thickness of fillet.

Toss the basil with the lime juice and top each serving of curry with a pinch or handful to taste.