For best results, use ripe but firm heirloom tomatoes. Their varied colors and flavors add a unique touch to the salsa. If you don't have an outdoor grill, a grill pan or even a hot skillet can be used. Feel free to add a pinch of red pepper flakes to the spice rub for an extra kick. The salsa can be made ahead of time and stored in the refrigerator for up to 2 hours. Be sure to add the avocado just before serving to prevent browning.