Prepare the Filling: In a medium bowl, combine the drained and rinsed white beans, pesto, salt, pepper, 1/2 minced red onion, and minced celery.
Mix gently until the beans are well-coated.
Taste and adjust seasoning if needed.
Cover and chill in the refrigerator for at least 1 hour to allow the flavors to meld.
Prep the Tomatoes: While the filling chills, slice about 1/2 inch off the top of each tomato.
Using a small spoon or melon baller, carefully scoop out the seeds and pulp, leaving a hollow shell. Invert the tomato shells on a paper towel-lined plate to drain any excess liquid.
Stuff and Serve: Once chilled, spoon the bean mixture evenly into each hollowed-out tomato.
Garnish with the remaining 1 tablespoon (15 ml / 0.5 fl oz) of minced red onion and serve immediately.