ONION TART - Lidia

Ingredients

• dough
• 3 cups all-purpose flour, plus more for rolling
• 1½ teaspoons kosher salt
• 2 sticks cold butter, cut into cubes
• 6 tablespoons cold water, plus more as needed
• onions
• 4 tablespoons butter, plus more for the cookie sheet
• 3 pounds onions, halved and thinly sliced
• 2 teaspoons kosher salt
• 1 cup heavy cream
• ½ cup grated Grana Padano cheese