Place one corn on the cobb onto a piece of aluminum foil, add 2 sprigs of fresh rosemary on either side and roll tightly to cover corn. Repeat with the remainder of the corn.
Grill corn in the foil on medium-high heat, turning frequently to cook evenly until cooked to desired texture, around 15-20 minutes. Remove foil packets from the grill. Remove corn from the foil and allow to cool to room temperature.
Slice the corn off the cobb with a sharp knife into a medium sized bowl. Add the onions, avocadoes and tomatoes.
In a small bowl, whisk together the remaining ingredients. Pour over corn mixture and mix until all ingredients are well blended. Serve!