To make the sauce, preheat a medium skillet over medium heat. Add the bacon and cook, stirring occasionally, until the fat melts and the bacon begins to brown, about 6 minutes. Add the onions and cook, stirring frequently, until they turn caramel colored, about 8 minutes. Add the flour, stir until it thickens into a paste and cook for 1-minute more to remove any starchy flavor. Add the beer and milk, whisk to make a thick sauce and bring it to a simmer. Season the sauce with salt and pepper and taste for seasoning. Cover the pan and keep it over low heat while you prepare the steaks.
Working with 1 steak at a time, put it into a gallon-size resealable heavy-duty plastic bag but do not seal the bag. Pound the steak with a meat mallet or heavy rolling pin until the steak is an even 1/4 inch thick. Remove the steak and re-use the bag to pound the remaining steak.
Mix the flour with the salt and pepper in a wide, shallow dish. Dredge the steaks in the flour to coat them well, and gently shake off the excess. Line a rimmed baking sheet with a wire rack and preheat the oven to 250°F. Heat the oil and butter in a large skillet over medium-high heat. Cook the steaks 1 at a time until golden brown, 1 1/2 to 2 minutes per side. Put them on the rack and keep warm in the oven. Repeat with the remaining steaks.
Slip the steaks into the sauce and cover them with it. Serve them immediately, or to hold them, turn off the heat and cover the pan to keep them warm until you're ready to serve.
Source: Curry, Lynne. "Steaks Smothered in Bacon-Stout Sauce." Pure Beef: An Essential Guide to Artisan Meat with Recipes for Every Cut. Pennsylvania: Running Press Book -Publishers, 2012. 184-185. Print.