In a small bowl, melt the butter halfway in the microwave for about 40 to 60 seconds. It should be mostly melted with a few soft unmelted pieces remaining.

Stir in the pesto, Dijon mustard, minced onion, and red pepper flakes. Set aside at room temperature to thicken while you assemble the sandwiches.

Preheat your oven to 350 degrees F.

Using a sharp serrated knife, slice the entire package of rolls in half horizontally without separating them into individual rolls. Place the bottom slab on a rimmed baking sheet or in a 9x13 inch pan.

Stir the mayonnaise and sugar together in a small bowl. Spread the mayo mixture evenly over the cut side of the bottom rolls.

Layer the Canadian bacon evenly over the bottom rolls, then place the Havarti cheese slices on top to cover fully.

Set the top slab of rolls back in place. Spoon and spread the pesto butter mixture evenly over the entire top surface, reaching all the way to the edges.

Cover the pan tightly with aluminum foil sprayed with nonstick spray on the underside. Bake at 350 degrees F for 12 minutes.

Remove the foil and continue baking for 2 to 5 more minutes until the tops are golden brown. Watch the bottoms closely to prevent over-browning.

Use a sharp knife to cut into individual sliders and serve hot immediately.