First, prepare the sauce. Melt the butter with the oil in a medium-sized pot over medium heat. Add the garlic, ginger, tomato paste, garam masala, and red curry powder and toast for 4 minutes, stirring every minute. Add the tomatoes, water, and salt and allow to simmer over low heat for 1 hour, stirring every 10 minutes. Remve the mixture from heat, puree it in a blender or food processor, and pour it back into the pot. Stir in the cream until incorporated and then cover and keep warm until serving time.