In a small bowl, combine the lemon juice with the salt and pepper. Slowly pour in the oil, whisking to emulsify.
Place the fonio in a large bowl and add the parsley, mint, mango, onion, tomatoes, and cucumber. Toss well and generously fold in the vinaigrette to taste. (You may have some left over.) Top with the spiced cashews (if using) and serve immediately.