Mix the flour and butter to make white roux for thickening the sauce. Set aside
Sauté the onion and garlic in butter then add the milk. Low heat to avoid boiling.
Add salt, pepper, lemon, parmesan cheese. Mix well
Add the white roux to make a thickened sauce and stir well.
If the sauce is already thickened use strainer to sift the roux.
Lastly add chopped parsley.
Served with your favorite cordon bleu chicken.