Halve and peel onion. Slice halves into thin strips. Trim and cut celery into 1/4" dice. Halve baguettes lengthwise. Use your fingers to hollow out baguettes slightly to make a 1/4" deep trench. Rinse chicken breasts, pat dry, and season both sides with 1/2 tsp. salt and 1/4 tsp. pepper.

Add macaroni to boiling water and cook until tender, 7-9 minutes. Drain in colander and run cold water over pasta until cool. Shake colander to remove as much water as possible, then transfer pasta to a medium mixing bowl. Add mayonnaise, seasoning blend, celery, and 1/4 tsp. pepper to bowl and toss to combine. Refrigerate pasta salad until plating. While pasta boils, cook chicken.

Heat a medium non-stick pan over medium-high heat. Add 2 tsp. olive oil and chicken to hot pan and cook until browned, 2-4 minutes per side. Transfer chicken to baking sheet and roast until chicken reaches a minimum internal temperature of 165 degrees, 4-5 minutes. Let rest 5 minutes before cutting into 1/3" slices. Reserve pan; no need to wipe clean. While chicken roasts, caramelize onions.

Return pan used to sear chicken to high heat. Add 2 tsp. olive oil and onions to hot pan. Season with 1/2 tsp. salt and 1/4 tsp. pepper. Cook, stirring constantly, 3 minutes. Reduce heat to medium and cook, stirring occasionally, until onions are golden brown and tender, 6-9 minutes. Transfer onions to a plate, wipe pan clean, and return to medium heat.

Brush baguette insides with BBQ sauce. Assemble panini in this order: bottom half of roll, onions, chicken, cheese (halve to fit roll), red cabbage, and top roll. Lightly coat tops and bottoms of panini with cooking spray and place in pan used to cook onions. Place a heavy pan or pot over panini and cook until crisp and toasted, 1-3 minutes. Repeat on second side. Remove to cutting board and halve on an angle.

Place panini on plate and serve pasta salad on side.