Combine first 3 ingredients in a shallow bowl; dredge beef in flour mixture. Heat the oil in a large Dutch oven over medium heat; add beef, fennel, shallots, and garlic, and sauté 5 minutes or until meat is browned. Stir in bell pepper and Madeira, scraping pan to loosen browned bits. Add escarole, mustard, and thyme; cover, reduce heat, and simmer 1 hour, stirring occasionally.
While beef simmers, place potato in a saucepan; cover with water, and bring to a boil. Reduce heat, and simmer 20 minutes. Drain. Return potato to saucepan. Add milk and 1/2 teaspoon salt, and beat at medium speed of a mixer until smooth. Serve beef mixture over mashed potatoes.