Pre heat oil in a pan to a medium heat (1/4 inch deep).
Place 1 chicken breast (and a little water) in the bag and seal it up, pushing out excess air. Use the bottom of a heavy pot or pan and pound each breast until flat, just shy of busting out of the bag. Repeat with the other 3 chicken breasts.
Blend pretzels in a food processer until fine.
Mix ground pretzels, thyme and ssome salt and pepper in a shallow dish (like a dinner plate)
Beat eggs and pour into a second dinner plate with a splash of water added.
One at a time coat each fillet in egg and then ground pretzels.
Add chicken to pan (do 2 batches if necessary - about 4 minutes on each side or until cooked through).
To make cheesy mustard sauce sauce
Heat butter in a pan over medium heat.
Add flour and cook for 1 minute.
Whisk in milk and when it starts to thicken add cheese and mustard and stir (figure of 8s!)
When it has melted completely its ready - drizzle over chicken and serve with lots of veg or salad.