Lemony Escarole Salad with Peaches and Feta

1/4 cup plus 1 tablespoon extra-virgin olive oil 3/4 teaspoon finely grated lemon zest 1/4 cup fresh lemon juice 1 garlic clove, finely grated Pinch of sugar Kosher salt Freshly ground black pepper 2 medium heads of escarole, inner light green and white leaves only, torn into bite-size pieces 2 large peaches—halved, pitted and thinly sliced 2 cups yellow cherry tomatoes, halved 2 Persian cucumbers, thinly sliced crosswise 1/2 small red onion, halved and thinly sliced 6 ounces feta cheese, crumbled (about 1 1/2 cups) Ground sumac (see Note) and/or dried oregano, for garnish (optional)