How to Make It
Place potato in a saucepan over high heat; cover with cool water. Bring to a boil; cook 10 minutes or until very tender. Drain. Return potato to pan; keep warm.
While potato cooks, place onions in a microwave-safe bowl; cover with a paper towel. Microwave at HIGH 4 minutes. Finely chop 1 (2-inch) cube of tenderloin, and set aside. Pat dry the remaining beef cubes with a paper towel; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper.
Heat a 12-inch cast-iron skillet over medium-high heat. Melt 1 tablespoon butter in pan; swirl to coat. Add seasoned beef cubes; sauté 3 minutes, turning to brown on all sides. Remove beef from pan. Add finely chopped beef and bacon to pan; sauté 3 minutes, stirring occasionally. Add mushrooms; sauté 5 minutes or until mushrooms brown, stirring occasionally. Add garlic, chopped thyme, and sugar; sauté for 1 minute, stirring constantly. Add tomato paste; cook 1 minute, stirring constantly. Add wine, and bring to a boil, scraping pan to loosen browned bits. Cook 2 minutes or until liquid is reduced by half.
Add onions and broth; bring to a boil. Cook for 1 minute. Combine 2 tablespoons water and flour in a bowl, stirring with a whisk until smooth. Add flour mixture to pan; cook for 1 minute, stirring constantly. Return browned beef cubes to pan; cook 2 minutes or until thoroughly heated and cooked to medium-rare or desired degree of doneness.
Add the remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, remaining 2 teaspoons butter, and milk to potatoes in saucepan; mash with a potato masher until desired consistency. Serve beef and sauce over potatoes; garnish with thyme sprigs, if desired.