Peel and remove pit from mango and avocado. Dice into 1/2-inch (1-cm) pieces and place in a bowl. Slice jalapeno in half and discard seeds. Finely mince half the jalapeno and add to bowl along with red onion. Stir in lime juice and salt.
Just before serving, stir in mint and peanuts. Taste and add more minced jalapeno and salt if you wish. Salsa will keep well up to 2 hours, but for best results stir avocado, mint and peanuts in just before serving.