Melt the butter in a saucepan over medium heat. Add the flour and cook, stirring, until the mixture bubbles. Gradually add the milk, in 3 batches, stirring constantly. Cook and stir for 5 mins or until sauce boils and thickens. Remove from heat. In the same pan, add the chicken and cook until brown. Then, add onions, potato, peas, lemon zest, lemon juice, thyme, parsley and salt. Cook until translucent and fragrant. Set aside until cooled to room temperature.