1. Cut a small slice from the top of each tomato; scoop out pulp with a small spoon or melon baller, and discard pulp. Place tomatoes, cut sides down, on paper towels, and let drain 15 minutes.
2. Meanwhile, combine avocado and lemon juice in a small bowl, stirring gently; drain. Stir together mayonnaise, bacon, and green onions; add avocado mixture, and stir gently until combined.
3. Spoon avocado mixture evenly into tomato shells. Cover with plastic wrap, and chill 1 hour. Sprinkle with salt and pepper to taste just before serving.