Fresh curry leaves, found in Indian markets and specialty produce stores, add a tart, aromatic flavor to this mix. They may be omitted, if unavailable.
Heat the oil in a small skillet set over medium heat until very hot but not smoking, about 1 minute. Add the mustard and coriander seeds, and heat until mustard seeds crackle, about 2 minutes. Add the curry leaves; stir to combine. Add cashews, almonds, turmeric, and cayenne pepper. Increase heat to medium high. Use a flat spatula to stir and turn over the ingredients, combining the flavors and toasting the nuts evenly, about 10 minutes.
Remove from heat, and add the lemon juice, cilantro, and salt. Nuts may be served warm or at room temperature. The mix may be made a day in advance and kept at room temperature in an airtight container.