Cut a thin slice from the top and bottom of each tomato. Using a knife or small spoon, carefully cut/hollow out tomato guts. Line a baking sheet with paper towels. Invert tomatoes onto the towels. Let stand for 30 minutes to drain.
Meanwhile, for filling, in a food processor, combine avocado, cream cheese, pesto and lemon juice. Cover and process just until combined.
Place tomatoes on a serving platter. Spoon filling into the tomato cups. If desired, cover loosely with plastic wrap and chill for up to 4 hours. If desired, sprinkle with snipped basil before serving.