Place mustard greens in a large Dutch oven. Add water, and bring to a boil. Reduce heat, and simmer 15 minutes; drain. Set aside.
Cook bacon until crisp, reserving drippings. Crumble bacon, and set aside.
Add oil to reserved drippings to yield 1/4 cup; pour into a large skillet. Add flour, mixing well; cook over medium heat, stirring constantly, 10 minutes or until roux is the color of a copper penny. Add onion; sauté 3 minutes or until tender. Stir in mustard greens, green pepper, salt, and pepper. Cover and simmer 1 hour. Stir in reserved bacon.