Heat a medium saucepan to medium-high heat and melt butter. Add onion, ginger and spices Stir frequently 3-4 minutes, or until lightly browned and spices are fragrant. Add remaining ingredients; season with salt. Bring to a boil then reduce heat. Simmer about 20 minutes, or until potatoes are almost tender. Increase heat slightly until potatoes are tender and sauce has thickened, about 5-10 minutes longer. Adjust seasoning to taste and serve hot over steamed basmati rice or with naan bread.