In a simple saucepan over medium heat, melt butter.
Sauté onions, chili pepper, ginger and garlic in the melted butter for 8-10 minutes until onions are soft and slightly browned.
Add the spice blend to the onion mixture and cook for 3 more minutes.
Stir in water and tomato paste, breaking up any lumps with a whisk.
Add sugar and salt, bring to a simmer (it happens quickly), then remove from heat.
Stir in heavy cream.
Use an immersion blender to blend the sauce until smooth. If using a countertop blender, blend for 30-60 seconds until creamy. If desired, add a couple more tablespoons of butter for extra richness. Cover and set aside.