On large baking sheet, spread out the asparagus and red onion, and add the pesto. Bake for 10-15 minutes (depending on thickness of asparagus), until the asparagus is tender.
Remove from oven and allow to slightly cool. Plate the asparagus and top with tomatoes, mozzarella, and fresh-cracked pepper and salt. (Optional to sprinkle pistachio nuts on top :)