Lightly season both sides of the fish fillets with salt and pepper. Set aside.
Heat a drizzle of oil in a large pan over medium heat. Add the sliced onions and sauté for 2–3 minutes, until softened and slightly translucent.
Add garlic and ginger, cooking until fragrant, about 30–60 seconds.
Stir in the Thai red curry paste and cook it for 1–2 minutes. This step deepens the flavor and removes any raw spice taste.
Pour in the chicken broth and coconut milk, stirring until smooth and combined. Bring to a gentle simmer.
Add fish sauce, sugar, and lime juice. Taste and adjust as needed—more lime for brightness, fish sauce for saltiness, or sugar for balance.
Gently slide the fish fillets into the simmering curry. Spoon some sauce over the top, then cover and let cook for 6–10 minutes, depending on thickness, until the fish is opaque and flakes easily.
Top with fresh cilantro and sliced Thai chilies. Serve hot over steamed rice, spooning plenty of curry sauce over the top.