Preheat the oven to 350 F. Cook pasta according to package directions. Drain and place into a large bowl. In a large saucepan over medium heat, melt butter.
Add in flour and ranch seasoning mix, and cook for a minute to brown. Pour in milk, and 2 cups shredded cheddar cheese. Continue to cook over medium heat, while whisking until the cheese has fully melted and the sauce has thickened.
Mix the sauce with the cooked pasta. Spread the mixture into a greased baking dish. Top with the remaining 1 cup shredded cheddar cheese and fried onions. Bake for 25-30 minutes.
