Mangu

Ingredients

• 4 to 5 large green plantains, peeled, brown spots and blemishes scraped off, cut in half, halves sliced down the middle lengthwise
• Salt
• 3 tablespoons butter
• ¾ cup ice-cold water, divided
• 1 small red onion, cut into ¼ inch slices
• ¼ cup white vinegar
• ¼ teaspoon salt
• ¼ teaspoon sugar
• 1 tablespoon olive oil