Add the beef and lamb and stir, crumbling and breaking them up with a wooden spoon till no longer pink. This may take up to 10 minutes. Drain off any fat.
In another medium saucepan, melt the butter over med-low heat, then add the flour. Whisk these two together and cook for 2 minutes, until all the 'rawness' has been cooked out of the flour.
Cut it only when it has cooled, (ideally overnight in the refrigerator) and warm the pieces before serving. It freezes well. Freeze before adding the final amount of parmesan cheese for baking.
If I am not making this to feed a crowd, then I will automatically divide everything into two dishes. One for freezing and one for eating!
If baking from fridge or freezer (after thawing), cover in tin foil, and bake at 350F this way for the first 20 minutes or so. Then remove the foil and bake for another 10-15 minutes until golden and bubbling.
Adapted from The Greek Gourmand