Whisk together the lemon juice and zest, honey, parsley, garlic, and oil. Pour into a large resealable baggie.
Season the chicken with salt and pepper and add to the bag. Seal and toss to coat.
Place in a large bowl or on a sheet pan in the fridge for 15 minutes.
If you are grilling the sandwiches, heat your grill or otherwise heat a large skillet over medium-high heat.
Working in batches if needed, shake off excess marinade and cook or grill the chicken 5-6 minutes per side, then flip until done. Remove to a plate and let rest while you make the gravy.
For the gravy
Melt butter in a sauce pot over medium heat.
Whisk in the flour and cook for 1 minute, stirring constantly.
Slowly whisk in the chicken stock. Bring to a boil then reduce to a simmer. Cook until thickened, 5-7 minutes, stirring often.
Season with salt and pepper.
Remove from heat and stir in the mustard.
For the sandwich
Slice bread lengthwise.
Using tongs, grab the chicken and then dip it into the gravy. Place onto bread.
Serve with additional gravy on the side for dipping.