Penne alla Vodka

INGREDIENTS

• Grated Parmesan, for serving
• 1 cup vodka
• 1 whole medium onion, chopped finely
• 3 cloves garlic, minced
• 2 tablespoons olive oil
• 1/4 to 1/2 teaspoon salt
• 3 tablespoons butter
• 1 cup heavy cream
• Freshly ground black pepper
• One 14-ounce can tomato puree or tomato sauce
• 1 pound penne
• 1 generous pinch red pepper flakes, plus more if needed