Step 1

Combine fish, salt, chile powder, and turmeric in a medium bowl and, using your hands, gently toss fish to evenly coat in spices.

Step 2

Heat oil in a large nonstick skillet over medium-high. Working in batches to avoid crowding, cook fish pieces until lightly browned, about 2 minutes per side (the fish won’t be cooked all the way through). Transfer fish to a plate.

Step 3

Heat grapeseed oil in a medium Dutch oven or other heavy pot over medium-high. Add a few mustard seeds to pot. When seeds start to wiggle and pop, reduce heat to medium and add curry leaves (if using), asafetida (if using), and remaining mustard seeds. Cover pot and cook spices, swirling, until mustard seeds start popping more gradually, about 30 seconds. Uncover pot and return heat to medium-high. Add ginger and garlic and cook, stirring constantly, until beginning to brown, about 1 minute. Add chopped onion and a big pinch of salt and cook, stirring often and reducing heat if needed to prevent burning, until onion is golden brown and jammy, 7–9 minutes.

Step 4

Stir in turmeric, then tomatoes and bring to a simmer. Reduce heat to medium and cook, stirring often, until tomatoes are glossy and darkened in color and oil starts to separate around edges of pot, 5–7 minutes.

Step 5

Add coconut milk and broth and stir to combine; season with salt. Bring curry to a simmer, then reduce heat to medium-low. Cover and cook, stirring occasionally to prevent sticking, 10 minutes.

Step 6

Add fish to curry. Return to a gentle simmer and cook until fish is just cooked through and flakes easily with a fork, about 4 minutes. Taste and season with more salt if needed.

Step 7

Divide fish curry among bowls. Top with sliced red onion and cilantro. Serve with rice and lime wedges for squeezing over.Do ahead: Curry can be made 5 days ahead. Let cool, then transfer to an