1. Toss onion with 1 Tbsp. vinegar; set aside. Place cucumbers in a colander and toss with 1/4 tsp. salt. Drain for 10 minutes. Pat dry.
2. Mix onion and cucumbers in a large bowl. Toss with olive oil and remaining 2 Tbsp. vinegar. Add dill; season with salt and pepper. Cover and chill until ready to serve.