Preheat the oven to 375. 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.
In a medium saucepan, add cream of mushroom soup, milk, velveeta, and sour cream. Heat on medium-high, stirring constantly, until velveeta is melted.
Stir mushroom, thyme, and salt and pepper to taste into soup mixture. Mix into large bowl with pasta and beef.
Spread into baking dish.
In a small bowl, mix together the bread crumbs and melted butter. Mix in remaining cheddar. 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.