Start by preheating your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish. Bring a large pot of salted water to a boil, and cook the pasta until just al dente. This usually takes about 8 minutes. Drain the pasta and set it aside in a large mixing bowl.

Heat the vegetable oil in a skillet over medium heat. Sauté the chopped onions until soft and fragrant, which should take about 5 minutes. Add the ground beef and cook, breaking it up with a spoon or spatula, until it’s browned evenly. Drain off any excess grease, then transfer the beef-onion mixture to the bowl with the pasta.

To the pasta and beef mixture, add your condensed cream of mushroom soup, milk (measured using one of the empty soup cans), sliced mushrooms, and dried thyme. Stir everything together until well combined. Don’t forget to season with salt and pepper to suit your taste. Finally, fold in 3 cups of the shredded cheddar cheese, reserving the rest for the topping.

Spoon the mixture evenly into your prepared baking dish. For the topping, combine the dry bread crumbs, melted butter, and remaining shredded cheddar cheese in a small bowl. Sprinkle this cheesy breadcrumb mix evenly over the casserole.

Place the dish in your preheated oven and bake for 30-35 minutes. The casserole is done when the topping is golden brown and crispy. Remove it from the oven and allow it to sit for a few minutes before serving.