Finely zest one lime, reserving 1 teaspoon for the onions and setting aside the remainder for topping the salad later. Juice one tablespoon of lime into a small bowl. Thinly slice the red onion (about 1/2 cup) and add it to the bowl along with 1/4 teaspoon of the granulated sugar and 1/4 teaspoon of kosher salt. Combine thoroughly to coat the onions and allow them to pickle for at least 15 minutes, stirring once or twice. Juice the remaining 1 1/2 limes to yield 1/4 cup juice. Finely chop the garlic and enough cilantro leaves to pack 1/4 cup. In a separate bowl or jar, combine the lime juice, garlic, chopped cilantro, olive oil, remaining 1 teaspoon sugar, and remaining 1/2 teaspoon kosher salt; whisk or shake until emulsified. Tear the butter lettuce into bite-sized portions, adding about 10 packed cups to a large salad bowl. Thinly slice the cucumbers; halve the cherry tomatoes; pit and slice the peaches into 1/2-inch wedges; crumble the feta cheese and add all to the bowl. Scatter the pumpkin seeds over the top, followed by the pickled onions (discarding the pickling liquid), reserved lime zest, and extra torn cilantro leaves and stems. Season with kosher salt and freshly ground black pepper to taste. Drizzle half of the dressing over the salad, gently toss to coat, then finish with the remaining dressing.