Peel and grate the carrots using the large holes of a box grater.
Slice the cucumbers into thin half-moon pieces, about 1/8-inch thick.
Thinly slice the red onion. For milder flavor, soak sliced onions in cold water for 5-10 minutes, then drain and pat dry.
Combine cucumber, carrots, and onion in a large bowl.
In a separate small bowl, whisk together olive oil, lemon juice, chopped dill, salt, and pepper.
Pour the dressing over the vegetables and toss gently to coat everything evenly.
Refrigerate for at least 15 minutes before serving to allow flavors to blend.
If using optional ingredients, add them just before serving and toss gently to combine.