Gnocchi alla Sorrentina

Ingredients

• 4 russet or baking potatoes
• 1 large egg, beaten well (55 grams)
• 4 ounces all-purpose flour (115 grams)
• 1 ¼ teaspoons kosher salt
• 3 tablespoons extra virgin olive oil
• 1 small yellow onion, chopped
• 3 ½ cups canned whole San Marzano tomatoes, peeled
• 8 ounces fresh mozzarella, (small balls called ciliegine)
• Torn basil leaves
• ½ cup finely grated Parmigiano Reggiano