Can't find 3-inch avocados? Buy the smallest ones available and scale up the ingredients: a 1 oz. (28-gram) jar of roe will contain about 6 tsp.
Arrange avocado halves on a platter, hollows up. Fill each with a mound of yogurt (2 to 4 tsp.). Sprinkle with about 1/2 tsp. roe. Top with a few feathery dill fronds. Squeeze with lemon juice to taste and serve.