Sprinkle the chicken breasts with Dusted Chipotle BBQ Spice or other chipotle BBQ spice, to taste. Rub in with your hands.
Place mayonnaise into a small bowl.
Add Dusted Chipotle BBQ spice or other chipotle BBQ spice, to taste, and mix well. Adjust to taste by adding more mayonnaise or spice.
Spread chipotle BBQ mayonnaise on both sides of each onion roll.
Sprinkle a generous amount of Asiago cheese on both sides of each roll.
Turn oven on broiler.
Place the rolls under the broiler for about 5 minutes or until the cheese is melted in the bread is crispy.
Remove from oven and set aside and keep warm.
Place a skillet, large enough to hold all 4 chicken breasts (if you do not have one this large, cook 2 or even 1 breast at a time) over medium heat.
Add onion slices. Allow to cook for a few minutes until they start to soften and become fragrant.
Add chicken breasts. Cook, flipping as needed, until done to an internal temperature of 165°F.
Place one chicken breast on each prepared bun bottom.
Spoon grilled onions over each breast.
Dress each sandwich with blue cheese, bacon and spinach leaves.
Add top bun and serve warm, with more chipotle BBQ mayonnaise on the side for dipping, if desired.