Spray the olive oil cooking spray on a large nonstick skillet. Add the ham, onion, and asparagus and cook for several minutes, stirring frequently, until the asparagus is crisp-tender and the ham is beginning to brown. If the skillet becomes too dry, place a lid over the skillet periodically.
Spread each pita with a quarter of the farmer or goat cheese. Top each with a quarter of the asparagus mixture and 1 1/2 teaspoons of the Parmesan cheese. Spray the top of each tart with a light coating of cooking spray.
Bake on a baking sheet for 10 minutes until the cheese is melted and the tarts are just beginning to brown.