How to Make It
Combine yogurt and milk in a medium bowl, stirring well with a whisk until smooth.
Stir in grapes, cumin, sugar, and pepper. Combine oil and mustard seeds or cumin seeds in a small skillet over medium-high heat; cook until cumin darkens or mustard seeds crackle (1 to 2 minutes), stirring frequently. (Cover pan if using mustard seeds.) Add fennel seeds and curry leaves, if desired; cook, uncovered, 10 more seconds, stirring. Pour oil mixture over yogurt mixture; chill completely (about 1 hour). Stir in salt before serving.