In a very large stockpot, heat oil over medium-high to high heat. Crumble in sausage and cook, 6 to 7 minutes, until browned, stirring occasionally. Add onion, garlic and Italian seasoning; reduce heat to medium and cook 4 minutes. Stir in kale and chicken broth. Cover and cook 8 minutes. Add 4 cups water, cover and bring to a boil over high heat, about 5 minutes. Add tortellini. Cook, covered, 3 minutes. Meanwhile, whisk flour with 1/2 cup water.