Whisk pesto and vinegar together in the bowl you'll be serving the salad in. Drizzle in olive oil slowly while whisking. Taste. Adjust to your preference (more vinegar? Oil? Pesto? Salt?)
using a mandoline very thinly slice squash, cucumber and onion into serving bowl over the pesto dressing. Toss gently with hands to make sure all vegetables are coated
Add halved cherry tomatoes and arugula and toss again.
Top with shavings of parmesan and a little cracked pepper, if desired.
Enjoy!
If you want to make this ahead, don't add the arugula or cheese until just before serving
Notes
summer squash are varieties of squash with thin edible skin and tender flesh. They don't need to be peeled.
If you don't have a mandoline slice vegetables by hand as thinly as possible or use a food processor with a thin slicing blade.
If you're using a larger cucumber slice it in half lengthwise first and scoop out the seeds so that your salad isn't watery
The same thing applies to the squash, if using larger summer squash scoop out spongy/seedy centers so that your salad isn't watery
Arugula should be added just before serving so that it doesn't get soggy