Mix the yogurt, mayonnaise, vinegar, sugar, salt, and pepper in a bowl and set aside.: When you whisk these together you should notice the dressing becoming glossy and slightly thick, with the scent of tangy Greek yogurt and the faint acidity of white vinegar coming forward; this mixture acts as the glue for the salad, ensuring every slice of cucumber is evenly coated. A smooth texture means the sugar has dissolved and the mayonnaise has emulsified with the yogurt, lending a velvety mouthfeel; if the dressing looks separated, whisk vigorously or add a tiny splash more vinegar to bring it back together. Avoid over salting here, because the salty bite of feta cheese will join later, and too much salt now can make the whole salad unbalanced. A common mistake is rushing the whisking, which can leave granules of sugar undissolved or an uneven texture, so take a moment to get it shiny and cohesive before you set it aside.
Add the cucumber, onion, dill, feta, and dressing to a large bowl and toss until well combined. Garnish with more dill or feta and serve.: As you fold everything together you should hear a gentle clinking as the crisp slices of English cucumber meet the bowl, and you will see the dressing cling to the edges of each slice, making them gleam; that shiny coating is the sign the salad will be flavorful in every bite. The thinly sliced red onion disperses small threads of sharpness throughout, and the chopped dill releases a fresh, grassy perfume as it warms slightly from your hands during tossing. When the feta cheese crumbles into pockets, it creates creamy, salty bursts that contrast with the cool cucumber . If the salad seems watery, it usually means the cucumber seeds were not removed or the dressing was added too early, so consider draining excess liquid before serving; a typical slip is adding the dressing immediately after slicing seeded cucumbers, which can dilute the dressing and flatten the flavors. Aim for an even distribution, tossing gently so you preserve the crunch and keep the dressing coating rather than pooling at the bottom.
