Altered ingredient amounts.
The first time I made this, I found that the sauce was a bit runny, even after adding the cheese, so when I next made it I increased the flour to 4 tablespoons. This time I also didn't have enough butter, so I used some sour cream to help make the roux (butter/flour mix) before adding the flour. I also found I had no lemon juice, so I used lime juice instead. I measured the salt and mustard powder, so that when I made this again I would know how I got the flavour we enjoyed. I used 1 tsp salt, and approximately 1/2 tsp of mustard powder.