Thinly slice the cucumbers into 1/8-inch rounds and place them in a large mixing bowl. Chop the scallions into 1/4-inch pieces and very thinly slice the red onion into half-moons. Add these to the bowl with the cucumbers along with the capers and roughly chopped fresh dill. This prep work ensures all vegetables are ready when you need them, and the dill's delicate leaves stay fresh until the final toss.
In a separate bowl, whisk together the sour cream, lemon juice, 1 tablespoon everything bagel seasoning, and 1/4 teaspoon garlic powder. Season to taste with salt and black pepper. The acidity from the lemon juice will help balance the richness of the sour cream while the garlic powder adds subtle depth without overpowering the fresh vegetables.
Pour the dressing from Step 2 over the vegetables in the large bowl and gently toss until everything is evenly coated. I like to sprinkle the second tablespoon of everything bagel seasoning and the red pepper flakes on top just before serving—this keeps them from getting soggy and ensures they add that signature crunch and spice that makes the salad special. Taste and adjust seasoning if needed.
Transfer the salad to a serving dish or individual plates and enjoy right away. The best time to eat this salad is within minutes of assembly, while the cucumber rounds are still crisp and the flavors haven't started to meld too much.