Prepare vegetables first. I recommend to cut onion and potatoes into large chunky pieces. Slice carrot about 1cm. Cut chicken fillets into 4 to 5cm chunky pieces.

Heat oil in a pot over a medium to high heat, cook chicken for a minute. Add vegetables, chicken stock and bay leaf, and bring it to the boil. Reduce heat to medium to low, and cook until vegetables are soft.

While you are cooking chicken and vegetables, make the roux. Heat butter in a saucepan over medium heat until melted and foaming, add flour and cook stirring until mixture bubbles. Slowly add milk, stirring constantly until it thickens. *Note: This process can be done in a heat-proof bowl in the microwave.

When vegetables are just cooked, taste the soup and season with salt & white pepper as required, then add the roux. Gently stir to let the roux dissolve, cook for a few more minutes over a low heat. Add extra milk or water to thin if required. *Note: If the soup happened to become too thin, I recommend to thicken it with a mixture of corn starch and milk OR water.