Place sliced cucumbers in a colander over a bowl. Sprinkle with 1 teaspoon salt and toss. Let sit for 15 minutes to draw out excess water.
Rinse cucumbers under cold water and pat dry with a clean towel.
In a large bowl, whisk together white wine vinegar, olive oil, Dijon mustard, honey, minced garlic, black pepper, and red pepper flakes if using.
Add cucumbers, sliced red onion, and chopped dill to the bowl. Toss well to coat.
Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.
Toss again just before serving. Taste and adjust seasoning if needed.