Peel and finely chop one 2″ piece ginger and 4–6 garlic cloves. (You should have about 2 Tbsp. each.) Mix 1 Tbsp. finely chopped ginger, 1 Tbsp. finely chopped garlic, 1 tsp. ground turmeric, 2 Tbsp. fish sauce, ½ tsp. crushed red pepper flakes, and a pinch of kosher salt in a small bowl. Cut 1 lb. white fish fillets, preferably skin-on, into 4 equal pieces, then massage marinade onto fish to coat evenly. Place in a shallow bowl or dish. Let marinate in the fridge at least 30 minutes and, preferably, up to 2 hours.