Place taco shells on baking sheet into a 325° oven and heat for 5 to 6 minutes. Remove and set aside.
In a large skillet over medium-high heat place the vegetable oil and onion, cook and stir for 2 to 3 minutes and then add squash and taco seasoning mix; cook an additional 2 to 3 minutes.
Add 1/4 cup turkey in each taco shell and add salt and pepper to taste. If desired, add salsa, additional squash or slices of avocado to the top.