1) In a bowl, mix together butter and flour.
2) Preheat oven to 350 degrees.
3) Take a plate and mix celery, onion, poultry seasoning, salt and sage together.
4) Take the fillet and coat in flour and then in egg mixture and then in celery mixture.
5) In a frying pan, heat some butter and roast the fillets on both the sides until golden brown.
6) Take a baking dish and put the fillets into it.
7) Bake for 15-20 minutes.
8) In the same pan add chicken stock, garlic and sage.
9) Add butter mixture and stir constantly to avoid lumps in the sauce. Let it thicken.
10) Pour the sauce on the turkey and bake again for 15 minutes.
11) Serve the crusted turkey breast with sauce immediately.