1. Whisk milk and potato flakes together in large bowl. Add beef, 1/2 teaspoon salt, and 1/2 teaspoon pepper and knead until combined. Shape into two 1/2-inch-thick oval patties. (Patties can be covered and refrigerated for up to 24 hours.)

2. Melt 1 tablespoon butter in 10-inch nonstick skillet over medium-high heat. Cook patties until well browned, 3 to 5 minutes per side. Transfer to plate.

3. Return now-empty skillet to medium-high heat, add onion and remaining 1 tablespoon butter, and cook until browned, about 5 minutes. Add mushrooms and 1/4 teaspoon salt and cook until liquid has evaporated, 5 to 7 minutes. Stir in flour and tomato paste and cook for 2 minutes. Slowly whisk in broth and port and bring to simmer. Return patties to skillet, cover, reduce heat to medium-low, and cook until meat registers 160°F, 8 to 10 minutes, flipping patties halfway through cooking.

4. Transfer steaks to platter. Continue to simmer sauce until slightly thickened, 2 to 4 minutes. Season sauce with salt and pepper to taste. Pour sauce over steaks and serve.

Serves 2