Saute onion in olive oil on medium high heat for about 3 minutes. Add squash and stir about once a minute for about 5 minutes. The squash should be tender, but not too brown. In fact many pieces will not be brown, but some will be light brown. Do not overcook. Taste one and see if they are done to your liking. Add salt to taste.
Warm the tortillas in the microwave for like 10 seconds until warm.
Spoon warm squash mixture onto tortillas down the middle. Add some shredded cheese. Add some chopped tomato. Wrap up and take a bite.