In a small bowl, melt the butter halfway in the microwave. It should be mostly melted and the un-melted parts should be soft. It shouldn't take more than 40-60 seconds.
Stir in the pesto, Dijon, minced onion, and red pepper flakes. Let this mixture sit at room temperature to thicken up (or stick it in the fridge) for a few minutes while you assemble the sandwiches.
Preheat your oven to 350 degrees F.
Use a sharp serrated knife to split the rolls in half (do no separate into individual rolls.) See photos. Place the bottom half of the rolls on a rimmed baking sheet. (Or you could also do it in a 9x13 inch pan.)
In a small bowl, combine mayonnaise and sugar. You can skip this step and just spread the mayo on if you want. I like it a little sweeter.
Layer the Canadian Bacon on the bottom half of the buns. Layer the cheese on top.
Top with the top half of the rolls.
Spoon the butter-pesto mixture over the tops of the rolls and spread it all over.
Cover the rolls with aluminum foil that has been sprayed with nonstick spray (so the cheese doesn’t stick to it).
Bake at 350 for 12 minutes.
Remove the foil. Continue baking for another 2-5 minutes, until the tops are browned. Keep an eye on the bottoms of the rolls so they don’t burn. I like them to be nice and crispy.
Use a sharp knife to cut each sandwich and serve hot!