In a small bowl, whisk together all dressing ingredients. Set aside.
In a large bowl, toss the sliced cucumbers (3) with 1 teaspoon salt, then allow to sit for 1 hour at room temperature to "sweat". Drain off liquid.
Add tomatoes (2 cups), Mozzarella (2 cups), onions (1 cup), and dressing to drained cucumbers and toss until ingredients are evenly coated. Cover and refrigerate for 1 hour before serving. Adjust seasoning and serve. Store leftovers in the fridge.
Notes
You’ll want to sweat your cucumbers with a little salt before mixing up your cucumber salad to pull out some of the moisture. This helps keeps your salad from being watered down after it sits in the fridge.
I recommend an hour of chill time in the icebox before serving but everyone has different tastes. My family loves this cucumber tomato salad immediately after it’s made while those cukes are super crunchy. Chill time will soften them just a little, but it also gives all those flavors time to mingle and soak in.