Heat 1 teaspoon oil in a Dutch oven over medium-high heat. Add leek; sauté 6 minutes or until tender and golden brown. Add garlic; sauté 1 minute. Spoon leek mixture into a large bowl.
Place 1/3 cup flour in a shallow bowl or pie plate. Dredge chicken in flour, shaking off excess. Heat remaining 1 tablespoon oil in pan over medium-high heat. Add half of chicken mixture; sprinkle with 1/8 teaspoon salt and 1/8 teaspoon black pepper. Cook 6 minutes, browning on all sides. Add browned chicken to leek mixture. Repeat procedure with remaining chicken mixture, 1/8 teaspoon salt, and 1/8 teaspoon black pepper.
Add wine to pan, scraping pan to loosen browned bits. Combine 1 cup broth and 1 tablespoon flour, stirring with a whisk until smooth. Add broth mixture, remaining 2 cups broth, water, and mustard to pan; bring to a boil. Stir in chicken mixture, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon black pepper. Cover, reduce heat, and simmer 30 minutes.
Stir in potato. Cover and simmer 30 minutes or until potato is tender. Stir in kale; cover and simmer 10 minutes. Garnish with crushed red pepper, if desired.