Preheat oven to 450 degrees. Place asparagus and onion in a lined 9x13 baking dish. Toss with a drizzle of olive oil, salt, and pepper. Roast for 30 minutes.
Spread pesto on half of bread slices. On the other halves, layer on sliced chicken, asparagus, onion, and mozzarella.
Toast under broiler or press in a panini maker.
Recipe by Rachel Schultz at https://rachelschultz.com/2019/02/25/pesto-asparagus-chicken-panini/