To save gritty and clumpy cheese sauces, add an acid like lemon juice or white wine to unwind the clumped-up protein molecules and prevent the sauce from seizing any further.
A squirt of lemon comes to the rescue of ruined sauces in a pinch. All you have to do is cool the broken sauce, add a tablespoon of lemon juice, and whisk vigorously.
This trick works best if you haven't already added acid to the sauce. Balance is the key, as you might get the opposite effect of further breakage if you add too much acidity.