Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a 9x13 inch baking dish. Bring a large pot of lightly salted water to a boil Add pasta, and cook until al dente, about 8 minutes; drain, and set aside in a large bowl.
Heat vegetable oil in a skillet over medium heat. Saute onions until they begin to soften. Add ground beef, and cook, stirring, until evenly browned. Drain off grease, and pour into the bowl with the pasta. Stir in the condensed soup, then measure the milk using the soup can. Add mushrooms, and thyme until well blended. Mix in 3 cups of the cheese, and season with salt and pepper. Spread into baking dish.
In a small bowl, mix together the bread crumbs and melted butter. Mix in remaining 1 cup cheese. Sprinkle the mixture evenly over the top of the baking dish.
Bake for 30 to 35 minutes in the preheated oven, or until topping is crispy and golden.
