Dice the 400g chicken breast fillet and season with salt and pepper. Fry in a pan over medium-high heat for 8-10 minutes, stirring occasionally, until cooked through (internal temperature 74°C / 165°F).
Cut the 2 hard-boiled eggs into large cubes.
Melt 2 tablespoons butter in a saucepan over medium heat, add 2 tablespoons flour and stir for 2-3 minutes until light golden. Slowly pour in 150 ml vegetable stock while stirring constantly to avoid lumps, then simmer for 10 minutes until thickened.
Stir in 3 tablespoons mustard and 50 ml heavy cream. Season with salt, pepper, and 1 tablespoon lemon juice.
Add the cooked chicken and egg cubes to the sauce and stir gently for 2 minutes until heated through.