Heat skillet over medium-high heat. Once warm add the olive oil.
Pour the veggies (omit spinach until later if you use it) in and stir so they are all coated in oil. Then leave it sit to allow them to start to caramelize.
Stir every 3-4 minutes until the onions start to turn translucent. Once they are cooked, then stir in the spinach and continue to cook until it is wilted. The vegetables should start to get charred look. That is when they are done.
Assemble tacos by heating the tortilla shells. Add the fajitas, guacamole, and any salsa or sour cream you may want to top it with. You can also add rice or beans but generally I like them a bit lighter.