Finely chop the onion and mince the garlic cloves, keeping them separate. Heat 2.5 tablespoons of butter in a large skillet over medium heat until it foams, then add the chopped onion and cook for about 3-4 minutes until it becomes translucent and soft. Add the minced garlic and dried thyme, cooking for another 30 seconds until fragrant. This aromatic base will form the flavor foundation for the entire dish.

Add the ground beef to the pan with the aromatics from Step 1 and cook over medium-high heat for 5-7 minutes, breaking it up as it cooks, until it's browned throughout. Drain excess fat if needed, then sprinkle the flour over the beef and stir constantly for 1 minute to create a roux base. Pour in the beef broth and milk while stirring, then add the salt, bouillon powder, black pepper, paprika, and dried oregano. Simmer for 3-5 minutes, stirring occasionally, until the sauce thickens enough to coat the back of a spoon—this creates a rich, cohesive filling.

While the beef sauce simmers, preheat your oven to 375°F. Slice the 7 medium potatoes into thin, even slices (about 1/8-inch thick)—I recommend using a mandoline slicer here for consistency, which helps them cook evenly and creates a more cohesive casserole texture. Set the sliced potatoes aside.

Lightly grease a 9x13 inch baking dish with butter or cooking spray. Spread half of the sliced potatoes in an even layer on the bottom of the dish, then pour half of the beef sauce mixture from Step 2 over them. Repeat with the remaining potatoes as the second layer, followed by the remaining beef sauce. Press gently to ensure the potatoes are evenly distributed and the sauce reaches throughout.

Cover the baking dish tightly with aluminum foil to trap steam, which helps the potatoes cook through evenly. Bake at 375°F for 45 minutes until the potatoes are fork-tender. I like to test one with a fork around the 40-minute mark to gauge doneness, as potato thickness can vary slightly.

Carefully remove the foil from the hot casserole and sprinkle the shredded cheddar cheese evenly over the top. Return the uncovered dish to the oven and bake for 10-15 minutes until the cheese is melted, bubbly, and just beginning to brown at the edges. This final step creates a golden, cheesy crust that adds texture and richness to the dish.

Remove the casserole from the oven and let it rest for 5 minutes before serving—this allows the layers to set slightly, making it easier to scoop and plate. Garnish the top with fresh chopped parsley for a bright finish and pop of color.