With sharp knife, cut 1/4-inch off stem side of each tomato and 1/8-inch off bottoms. Using melon baller or small spoon, remove insides of tomatoes; discard.
Make cuts in ends of each green onion piece; place in bowl of ice water until curly.
In small bowl, mix chicken salad and horseradish. Place 1 teaspoon chicken salad in each tomato. Place 1 green onion brush in top center of each tomato for stem.