Fill a large pot with water and bring to a boil. Add a handful of salt and pasta and boil, stirring occasionally, until pasta is fully cooked. Drain pasta, reserving 3 tablespoons pasta water. Set aside.
In a large saucepan combine evaporated milk, pasta water, egg, dry mustard, salt, and pepper and whisk together. In a large mixing bowl combine cheese, starch and lemon juice and toss together.
Place saucepan over medium-low heat and whisk together for about 2 minutes. Whisk in small handfuls of the cheese mixture at a time, making sure not to add more cheese until the previous addition had been fully incorporated to the sauce.
Continue to whisk and add cheese over the low heat until all the cheese has been added and a creamy and smooth sauce forms (make sure not to rush this process or your cheese sauce will not be smooth).
Once the sauce has been made, stir in the pasta, broccoli and pancetta. Adjust seasonings and serve.