Creamy Sausage & Kale Soup with Potatoes

Ingredients

• 1 1/2 lbs Italian sausage (mild or a mix of mild and hot)
• 1 large onion, chopped
• 1/4 to 1/2 teaspoon crushed red pepper flakes
• 2 teaspoons kosher salt
• 4 cloves garlic, minced
• 2 tablespoons all-purpose flour
• 4 cups chicken broth
• 3 cups whole milk
• 2 lbs russet potatoes, sliced into 0.25-inch thick coins
• 1 large bunch kale (about 5 cups), stems removed and leaves chopped
• 2 cups heavy cream
• 1/2 cup Parmesan cheese, shredded
• Salt
• black pepper
• Extra Parmesan cheese, for garnish