In a skillet, toast the cumin seeds over moderately high heat until fragrant, 30 seconds; transfer to a mortar and let cool. Add the salt and grind to a coarse powder.
In a large bowl, toss the tomatoes, cucumbers, onion, oil and lemon juice. Sprinkle 2 teaspoons of the cumin salt over the salad and toss. Serve the salad on a platter.