Prepare the ingredients by dicing the onion, mincing the garlic, grating the ginger, and slicing the red chili. Set them aside.
Heat the vegetable or coconut oil in a large skillet over medium heat until shimmering.
Add the diced onion to the skillet and cook for about 3 minutes until softened but not browned.
Stir in the garlic, ginger, and red chili, cooking for another minute until fragrant.
Pour in the coconut milk, lime juice, and brown sugar. Stir gently to combine.
Season the sauce with salt, pepper, and optional fish sauce. Let it simmer on low heat while preparing the fish.
Pat the grouper fillets dry, then season both sides with salt and pepper.
In a separate frying pan over medium-high heat, add a small amount of oil and cook the fillets for 3 to 4 minutes per side, until opaque and flaky.
Transfer the cooked grouper to the skillet with the coconut sauce and warm together for 1 to 2 minutes.
Serve the fish with sauce spooned over, garnished with chopped fresh cilantro.