How to Make It
Bring 2 1/2 cups water to a boil in a medium saucepan over high. Place red onion slices in a fine wire-mesh strainer over a bowl, and pour boiling water over onions. Let drain.
Stir together vinegar, sugar, and 1 teaspoon salt in a saucepan. Bring to a boil over medium-high; remove from heat.
Transfer drained onions to a clean jar with lid; pour hot vinegar mixture over onions. Use a spoon to press onions down so they are submerged in liquid. Seal jar, and let stand 30 minutes. (Refrigerate up to 1 week.)
Preheat oven to 400°F. Place potatoes in a large saucepan, and add cold water to cover. Bring to a boil over high. Reduce heat to medium-low; cover and simmer until a knife slides into potatoes with ease, about 20 minutes. Drain potatoes; let cool.
Stir together 1/4 cup cilantro, 3 tablespoons parsley, 3 tablespoons oil, garlic, paprika, cumin, and remaining 3/4 teaspoon salt in a large bowl. Add potatoes; using your hands, toss to coat potatoes with cilantro mixture.
Arrange potatoes on an aluminum foil–lined baking sheet; firmly press each potato to smash it, being careful not to break it. Bake at 400°F until crispy and golden, 25 to 30 minutes.
Stir together lemon juice, cayenne, remaining 1/4 cup cilantro, remaining 2 tablespoons parsley, and remaining 1 tablespoon oil. Spoon over potatoes; top with drained pickled onions.