The sauce can be refrigerated for up to 3 days, or frozen for up to 1 month.
4.5 servings cups
2 medium yellow onions, diced (2 cups)
3 cups water
1 cup unsalted, chopped cashews
1 cup whole-milk plain yogurt
2 tablespoons canola oil
2 tablespoons homemade or store-bought ginger-garlic paste (see NOTE)
Combine the onions and water in medium saucepan over medium-high heat.
Bring to a boil, then cover and cook for about 10 minutes, until the onions have softened.
Transfer the onions and their cooking water to a blender.
Remove the center knob of the lid so steam can escape.
Add the cashews and puree until smooth for at least 1 minute, starting on a low speed and gradually increasing to high.
Pour into a bowl, then stir in the yogurt.
Heat the oil in a large saucepan over medium heat.
Once the oil shimmers, stir in the ginger-garlic paste and cook for 30 seconds, stirring constantly, then stir in the yogurt mixture.
Cook for about 20 minutes, stirring often; about halfway through, the mixture will begin to sputter so keep an eye on it, and keep stirring to avoid scorching, to form a thickened sauce.
The korma sauce is ready to use, or cool completely before storing.
To make about 2 cups of ginger-garlic paste, combine 1/2 cup chopped (unpeeled) fresh ginger root, 8 ounces (1 1/2 cups) garlic cloves and 3/4 cup water in a blender; puree on HIGH until smooth. Transfer to an airtight container; seal and refrigerate for up to 1 week, or freeze for up to 3 months.
Adapted from "Rasika: Flavors of India," by Ashok Bajaj and Vikram Sunderam, with David Hagedorn.