Preheat the oven to 400° F. Line a large baking sheet with parchment paper.
In a large skillet over medium heat toast the panko crumbs until lightly golden, stirring constantly. Remove from the pan and allow to cool.
In a food processor pulse the pretzels and walnuts until coarsely ground. Place in a medium bowl and mix with the panko crumbs.
Add the olive oil, mustard, honey, and water to the food processor and pulse until smooth. Season with salt and pepper to taste.
Pound the chicken breasts to an even thickness (about 1/2") between two sheets of waxed paper using a meat mallet.
Pour half of the dressing into a large shallow dish. Place the flour and half of the pretzel mixture in two more separate shallow dishes. Add additional pretzel crumb mixture as needed.
Dredge the chicken in the flour then dip into the dressing, allowing excess to drip off, and finally into the pretzel crumbs. Press gently on breasts to help the coating adhere. Place on prepared baking sheet.
Bake 20 - 25 minutes depending on their thickness/size or until cooked through (internal temperature 165 - 174 degrees F.).
Let the chicken sit 5 minutes before serving with the remaining honey mustard dressing.