Heat a heavy 10-inch skillet over medium heat. Add almonds and pistachios and cook 8 to 10 minutes, stirring often. Let cool 5 minutes. Place in a food processor or heavy-duty blender and process until nuts are powdered. Set aside.
In a medium saucepan, heat butter over medium-high heat. Saute sweet onion until lightly browned. Add coriander, mace, white pepper, cardamom, cayenne pepper, and nutmeg. Cook about 1 minute, until fragrant. Continuing to stir, add saffron, cream, salt, and powdered nuts and bring to a boil. Lower heat and simmer, stirring occasionally, until sauce is thick enough to coat the back of a spoon, about 12 to 15 minutes.
Makes enough sauce for about 1-1/2 pounds of grilled meat or chicken breasts. Or simmer your favorite meatball recipe in the sauce for 15 minutes.