Drain tuna and set aside. Boil pasta in a separate saucepan.
In a saucepan, melt butter on a medium heat; once melted take off the heat, add half the flour and return to the heat stirring constantly. Remove from heat once it's all mixed through and add the remaining flour. Return to the heat and cook for 1-2 mins.
Remove from heat and add the milk gradually, whisking constantly the entire time to avoid clumps forming.
The mixture should be fairly runny at this stage so add about 1 cup of cheese (or more if you like it cheesy) the cheese will thicken the mixture so if it gets really thick just add a little extra milk.
Keep an eye on the pasta it should be done by now, so drain and set aside. Add lemon juice, mustard, salt, pepper and stir.
Fold in tuna and pasta to the sauce and stir through.
Pour into a casserole dish or medium sized pyrex dish and sprinkle with breadcrumbs.
Top with cheese and bake for 30-45 mins or until cheese is golden
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. - 11 Dec 2009