Preheat your oven to 375 Degrees
Line a roasting pan with foil and place your chicken breast being sure to leave at least 1" in between each piece. Cover with salt and pepper to taste. Roast chicken for 15 minutes then lower temperature to 325 degrees and roast until the center of the thickest part reaches 165 degrees.
Meanwhile; combine the flour, panko, 1 tsp Kosher salt, and 2 tbs seasoning salt in a large bowl and whisk together. Either using a mandolin or a very sharp knife, slice onions into very thing rings. Break onion rings apart and coat in flour mixture. Place coated onions onto a prepared baking sheet with non-stick spray.
Cook onions, turning them over every 10 minutes, for a total of 30 minutes. Remove any pieces that look done and continue to cook until 30 minutes is up.
While onions are cooking, slice baguettes into 1/2" slices and line on baking sheets.
Melt butter and apply atop baguette slices with a pastry brush.
Season baguettes with Italian seasoning and set aside.
Once chicken has reached temperature and rested for 10 minutes, slice across the grain into 1/4" slices and set aside.