Place the onion in a bowl. In a saucepan, bring the vinegar, sugar, mustard seeds, 1/2 cup water and 1 tsp. salt to a boil, stirring until the sugar and salt dissolve. Strain over the onion. Let cool; drain.
In another bowl, roughly smash the avocado with the back of a fork; season. Top the warm corn with the avocado, onion and black pepper.
Rachael Ray Every Day's July/August 2016 cover