Pat the fish very dry. Toss the chunks with the turmeric, 1 tbsp fish sauce, grated galangal, and yogurt if using. Let it sit for 15 minutes while you prep everything else.
Make the nuoc cham: stir the sugar into the warm water until it dissolves, then add the fish sauce, lime juice, garlic, and chili. Taste and adjust. It should be bright and a little sharp.
Heat the oil in a wide nonstick or cast iron pan over medium-high until it shimmers. Lay the fish in a single layer and leave it alone for 3 to 4 minutes so a golden crust forms. Turn each piece once and cook another 2 to 3 minutes until just cooked through. Work in two batches if your pan is crowded.
Pile in all the dill and scallions. Toss for 60 to 90 seconds, just until the herbs wilt and collapse around the fish but still look green. Kill the heat.
Scatter the peanuts over the top. Serve straight from the pan with the nuoc cham for spooning over, plus rice or rice vermicelli on the side.