Sauté chicken on Med High in the olive oil, using a Dutch Oven.
Make sure the bottom starts to brown, but NOT burn. Occasionally add a splash of sherry to prevent burning (the first 1/4) and deglaze the pan. You will start to get a nice sauce on the bottom. Sauté till chicken has a nice colour on the outside, but still raw on the inside. About 10 minutes.
Add the chicken broth, the last 1/4 of the sherry, whole onion pierced with the cloves, the celery, the carrot, the savory/bay leaves, and the salt/pepper.
Simmer for 2 hours on Low. It should be slightly simmering.
Once done, reserve 2 cups of the broth. Remove chicken and cut into pieces. Let cool. Discard everything else or use it for a soup.
Sauté the carrots, celery and mushrooms in the butter until softened in a Dutch Oven. About 10 minutes.
Add the chicken broth and simmer for 10 minutes.
Remove vegetables from the Dutch Oven.
Preheat oven to 350°.
Mix the flour with the milk. Slowly add this to the chicken broth and blend using a whisk. Heat slowly on Low. Stir continuously until the mixture has thickened to your preferred texture. Don't make it too runny. It should stick nicely to a spoon when dipped.
Add the chicken and vegetables back to the Dutch Oven, with the Sherry and Pimentos.
Mix together gently so as not to break up the chicken pieces.
Pour mixture into a casserole dish. Roll out the pasty and place on top. Place over the edge and press to the dish. Brush the milk on top of the pastry.
Place dish in oven and cook until pastry is golden brown. Between 40 and 50 minutes.