Parboiling and then steaming whole potatoes before you slice them guarantees salad-perfect spuds that are creamy and firm, not mushy. Curry powders vary greatly in heat, so you may have to add more when seasoning the sauce.
1. Place potatoes in large saucepan, and cover with water. Bring to a boil; reduce heat to medium-low; simmer 20 minutes, or until potatoes are just tender. Drain, and return to pan. Cover with kitchen towel, and let stand 20 minutes. Cool. Cut into ı/4-inch-thick slices.
2. Heat oil in skillet over medium heat. Add cumin and mustard seeds, and sauté 3 minutes, or until mustard seeds start to pop. Remove from heat.
3. Whisk together yogurt, milk, curry powder, and cumin seed mixture in large bowl. Add potatoes, and toss to coat. Season with salt and pepper, if desired. Chill 1 hour, or overnight. Garnish with chopped jalapeños, if using.