We discovered this at Stephen Oxaal’s Branch Line restaurant in Watertown, Massachusetts, where pickled mustard seeds take an avocado salad from simple to stunning. The pickling process takes just a few minutes and the result adds a tang and crunch that balance the lushness of the other ingredients. Conventional vinaigrettes —blends of fat and acid — tended to slide off the avocados. Instead, we eliminated the lemon juice from the dressing and drizzled it directly over the avocado slices, where it mingled with the dressing. We liked ricotta salata best, but Parmesan was a fine substitute.