Season all chciken breasts with salt and pepper to taste. Preheat a large braising dish or Dutch oven over medium-high heat. Add olive oil. When oil is hot, add chicken breasts and cook until browned on both sides, but not cooked-through, about 3 minutes each sides. Remove chicken from pan and set aside until later.
Reduce heat to medium and add sherry wine to scrape up the brown bits from the pan. Add onions and mushrooms and cook for 5 minutes. Add garlic and cook for another minute. Add the butter and let it melt before adding the flour. Mix together to form the roux. Cook for 2 minutes.
Reduce heat to medium-low. Mix half and half, chicken broth, seasonings, and Worcestershire sauce together in a large measuring cup. Slowly whisk liquid into roux, ensuring the roux doesn't break and stays thick. Add the chicken bouillon cube.
Bring mixture to a boil and then reduce to a simmer. Chop the cooked chicken into cubes and add back to the pan. Simmer uncovered for 10 minutes.
Preheat oven to 425 degrees. Bake frozen pastry shells as indicated on the packaging. Remove from oven. Remove the center rounds from each pastry shell and set aside.
Add the peas and diced pimentos to the pan. Simmer for another 5 minutes. Turn off heat. Let cool for 5 more minutes. Spoon into baked pastry shells and top with pastry rounds. Serve immediately.