Heat oil in a large ovenproof sauté pan over medium high heat. Add onions and salt; cook stirring frequently until onions are soft. Add asparagus and tomatoes; cook 5 minutes more, until veggies become tender.
In a medium bowl, beat eggs. Mix in cheese, then pour egg mixture over asparagus and allow to set for 1 minute. Bake in 450°F oven.
Cook for 10 minutes, until completely set. Cut into wedges and serve immediately.