Clean and wash the leek, quarter it lengthwise, and cut across into thin slices.
Melt some butter in a large pan over medium heat.
Add the sliced leek and white wine, season with salt and pepper.
Cover and cook for 5 minutes until leek softens.
While the leek cooks, cut the chicken breast fillets into bite-sized pieces, finely chop the fresh parsley, and cut the mushrooms into slices.
Add the chicken pieces, mushrooms, parsley, and cream to the pan and stir well.
Increase heat to medium-high and bring to a boil, then reduce heat to medium and simmer covered for 10 minutes, until chicken is cooked through (internal temperature 74°C / 165°F).
Season with lemon juice, salt, and pepper to taste.