There's not much that we wouldn't put this cheesy concoction on, but it's especially perfect mixed with al dente pasta, then baked with breadcrumbs on top.
In a large saucepan, melt butter over medium-high heat. Once the butter has melted, whisk in flour and cook until it begins to smell nutty and fragrant, about 45-60 seconds. Whisk in milk and stock, and continue whisking vigorously until flour and butter have dissolved. Bring to sauce to a bubble and thicken, about 3-4 minutes. Once sauce is thick, remove from heat and mix in mustard and cheddar cheese. Stir until cheese is completely melted.