A friend brought this to a backyard party once and I really liked it. She was kind enough to give me the easy recipe. I have made it several times and everyone seems to really enjoy the little morsels of coolness.
Cut a thin slice from the top of each tomato so you can scoop out the seeds. (I also cut a very thin slice from the bottom of each tomato so they stand upright.)
Using a small melon baller, carefully scoop out the inside of the tomatoes. Turn them upside down on a paper towel lined cookie sheet and let them stand for about 30 minutes to drain. You can either add the pulp into the filling mixture, or, if there are too many seeds, just discard the pulp.
Meanwhile prepare the filling...put avocado, cream cheese, garlic salt, pepper and lemon juice in a food processor. Cover and pulse until smooth.
Add the grated onion to the mixture and stir to blend. Spoon mixture into a piping bag with a round tip or a ziptop plastic bag (cut a small tip off of the plastic bag AFTER you spoon the mixture into it).
Place the tomatoes, open sides up, on your favorite serving platter. Pipe the filling into the tomato 'cups'.
Serve immediately or cover loosly and refrigerate for up to 4 hours.