Cut breasts in half lengthwise to make thinner pieces (or pound halves) and then cut them in half horizontally for smaller portions. See above photo.
Mix together mustard and soy in a bowl until combined.
Slather mixture over chicken pieces directly on the cutting board to avoid dirtying another plate.
Put bread crumbs on foil or parchment paper and take each piece of slathered chicken and coat with panko crumbs.
Heat oil on medium-high heat in the pan. Once heated, add chicken pieces in batches. For best browning results, do not crowd the pan.
Cook for 3 minutes on one side or until panko crumbs become browned.
Turn chicken and cook for another 2-3 minutes, until cooked through and browned.
Plate chicken and serve with your favorite sauce.
Notes
When working with raw chicken always use a separate cutting board and be sure to wash it and the counter thoroughly. I have a dedicated cutting board for chicken.If not following a gluten free diet, use regular soy sauce and panko crumbs.