Preheat the oven to 400 degrees F. Lay the bacon in a single layer on a rimmed baking sheet and bake until crisp, turning once, about 15 minutes. Drain on a paper-towel-lined plate. When cooled slightly, crumble the bacon into bite-size pieces. Reserve 1 tablespoon of the bacon fat from the pan.
Heat a two-burner stove-top grill pan on medium/medium-high heat. Oil the pan with 3 tablespoons of the canola oil. Liberally sprinkle the chicken breasts and onion rounds on both sides with salt and pepper. Carefully lay the chicken down on one end of the grill pan and the onions on the other. Cook the onions until charred on both sides and slightly softened, 6 to 8 minutes. Transfer to a cutting board. Cook the chicken until charred on both sides and an instant-read thermometer inserted into the thickest part of the breasts registers 160 degrees F, 18 to 20 minutes. Transfer to a cutting board. Rest the chicken for 5 minutes. Dice the chicken into 1/2-inch pieces and cut the onion into large bite-size pieces. Reserve.
Add the remaining tablespoon of canola oil and reserved bacon fat to a large saute pan over medium-high heat. Add peppers to the pan. Season liberally with salt and pepper. Cook, stirring occasionally, until the peppers start to brown around the edges and soften slightly, 2 to 4 minutes. Add the grilled chicken, grilled onion, garlic powder and cayenne to the pan. Stir to combine and cook, stirring occasionally, until the chicken and onion are warmed through, 1 to 2 minutes. Shingle the cheese on top of the chicken and vegetables, turn off the heat and cover the pan until the cheese is just melted, 2 to 3 minutes.
To assemble, spread 1 tablespoon of the mayonnaise on both sides of each toasted hoagie roll. Place 1/2 cup of the shredded lettuce, 4 to 5 slices of tomatoes and one-quarter of the chicken mixture on each hoagie. Sprinkle the crumbled bacon evenly among the sandwiches. Serve immediately with potato chips and pickle spears.