Using a slicer or a sharp knife, thinly slice the cucumbers and radishes.
Finely chop the green onions and dill.
In a small bowl, whisk together the Dijon mustard, olive oil, lemon juice, red pepper flakes, salt, and black pepper to make the dressing.
In a large bowl, combine the sliced cucumbers, radishes, green onions, and dill.
Pour the dressing over the top and toss gently to coat.
Serve immediately for a refreshing, flavorful summer salad.