If meal prepping to take to work, ensure the filling is completely cool before wrapping foil or baking paper. I would not prep anymore than a day ahead unless you are adding the lettuce fresh otherwise it will brown.

The key to ensuring cornflakes stick to the chicken is shaking off the excess egg prior to dipping in the cornflakes. If there is too much wet mix being tossed around with the cornflakes this will lead them to go soggy even before the chicken has touched them! I like to also keep one hand as a 'wet' hand for the chicken and my other as a 'dry hand'. Using one to dip the chicken in to the egg and the other to press in to the cornflakes. When pressing the cornflakes into the chicken, do one side, shake and then repeat with the other side. I find this ensures a full coating on the chicken.

Feel free to sub for store brought ranch dressing if you wish to save time!