Add the dressing ingredients to a large bowl. Whisk to combine.
Add the onions and mozzarella pearls and stir to combine.
Start your pasta water, and when it comes to a boil, add your pasta. (see notes below)
Cook the pasta according to the package instructions. (see notes below)
Quarter the tomatoes and add them to the bowl with the dressing.
Chop the parsley and add it to the bowl.
When the pasta is done, rinse it with cold water and drain it well.
Add the pasta to the bowl with the vinaigrette, tomatoes, and parsley.
Using a big spatula, gently mix the ingredients until well combined.
Taste and adjust the salt and pepper to your liking.
Enjoy immediately or chill in the refrigerator for 30 minutes before serving.
Notes
Do NOT cook pasta al dente for pasta salad. Cook it thoroughly, or it gets too firm as it sits in the fridge.
Bean pasta can be a little finicky, and every brand is different, so it can be tricky if you're using one for the first time.
Salt the pasta water, but not like the sea. The pasta absorbs the water, so too much salt is too much. You can add so much that the pasta itself becomes too salty, and the amount of salt I see people adding to the pasta water on the internet is absurd. I typically add about a tablespoon of kosher salt, which adds a very subtle salt flavor to the pasta.
I recommend rinsing the pasta. It is true that not rinsing it leaves the starch and helps the dressing stick, but I’ve also never had an issue with dressing sticking to the pasta. I prefer rinsing the excess starch off, and the cold water stops the cooking process, ensuring you don’t wind up with overcooked noodles.