Combine onions, ginger, chili pepper, and garlic into a mortar and pestle. Pound into a smooth paste.
Heat oil in a pot. Sauté prepared paste until aromatic.
Stir in the beef. When brown, add coconut milk and lemongrass.
Cook over moderate heat until beef is tender.
Stir in the sour cream, turmeric, salt, and pepper to taste.
Continue to cook until sauce thickens.
Happy cooking, Foodies!