Heat a large saucepan over medium high heat.  Add the oil and heat until shimmering.  Add the onions and cook, stirring frequently, until softened and beginning to caramelize.  Add the salt, pepper, mustard, and garlic.  Continue to cook until onions are incredibly soft, caramelized, and falling apart.

Add the pasta and the stock.  Bring to a boil and cook according to the instructions on the box.  When the pasta is cooked, most of the stock should be absorbed.  Stir in the cheese and any other additions.  Once combined, place in serving bowls, garnish if desired, and serve immediately.

This is best served right away, but leftovers can be stored in the refrigerator in an airtight container for up to 3 days and reheated in the microwave until piping hot, 1-2 minutes.