Step 1: Caramelize the Onions
Heat the butter and olive oil in a large skillet over medium heat.
Add the sliced onions, salt, and sugar. Stir to coat the onions evenly.
Cook the onions, stirring occasionally, for 20-25 minutes until they are golden brown and caramelized. Reduce heat if needed to prevent burning.
Add the beef broth and thyme, scraping up any browned bits from the skillet. Cook for 2-3 minutes until the liquid reduces slightly. Set the onions aside.
Step 2: Prepare the Beef Roll-Ups
Preheat your oven to 375°F (190°C). Lightly grease a baking dish with olive oil or non-stick spray.
Lay the beef slices flat on a clean surface and season with salt, black pepper, garlic powder, and smoked paprika.
Spread a small amount of caramelized onions over each slice of beef, followed by a generous sprinkle of mozzarella and Parmesan cheese.
Roll each slice tightly, securing it with a toothpick if needed, and place seam-side down in the prepared baking dish.
Step 3: Make the Onion Gravy
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk constantly for 1-2 minutes to create a roux.
Gradually pour in the beef broth, whisking to prevent lumps.
Stir in Worcestershire sauce, thyme, salt, and black pepper. Cook for 3-5 minutes, until the gravy thickens slightly.
Step 4: Assemble and Bake
Pour half of the onion gravy over the beef roll-ups in the baking dish. Reserve the remaining gravy for serving.
Cover the dish with aluminum foil and bake for 20 minutes.
Remove the foil, sprinkle additional mozzarella and Parmesan on top, and bake uncovered for another 10-15 minutes, until the cheese is golden and bubbly.
Step 5: Serve and Garnish
Drizzle the reserved onion gravy over the roll-ups before serving.
Garnish with fresh parsley or thyme for a pop of color and flavor.