Heat the butter in a saucepan over medium heat. Add the flour and cook, whisking, for 1 minute. Whisk in the wine and cook until slightly reduced, 1 minute. Whisk in the stock and simmer until thickened. Season with salt and pepper. Add the peas, heavy cream and pimientos and return to a low simmer. Add the tarragon, mustard and a few grates of nutmeg, then stir in the chicken.
Toast the bread, then liberally butter and cut corner to corner. Ladle the creamed chicken over the toast points and serve.
Cook's Note: The creamed chicken can be covered and refrigerated for a make-ahead meal. Reheat over medium heat, partially covered.
Put the chicken in a medium stockpot. Add the garlic, celery, bay leaf, carrots, lemons, onions and herb bundle and sprinkle with salt. Add enough water to cover the chicken. Bring to a boil, then reduce the heat to a simmer and cook for 45 minutes. Remove the chicken from the liquid and let cool. Strain the stock. Remove the skin and bones from the chicken, and shred the meat using your fingers or 2 forks.