Principles of full flavour in dishes
One of the most important things to consider when designing a dish is the balance of tastes in the dish: salty, sweet, sour, bitter, and umami.  Remember that certain tastes can combine with each other to cancel each other out, enhance one another, and sometimes even create new tastes.  Here are three rules of thumb that should help:


Sweetness can complement and even suppress bitterness.  As an example, think of beer.
Saltiness, in general, enhances the other tastes, and can often be masked by them.  Sweetness, for example, can suppress saltiness and, at the same time, saltiness can enhance sweetness.  That's why many baked confections contain salt.  Also, that's one reason why a classic pairing for salty feta cheese is sweet watermelon.
Sourness/acidity can make fatty foods seem less fatty.  That's one reason why fatty meats are often served with vinegar-based sauces.


It is important to think about what each ingredient contributes to the dish in terms of taste, flavor, texture, and color.  If two ingredients are serving the same purpose, then chances are you don't need one of them.  In your specific case, here is what some of your ingredients contribute in terms of taste:


Feta: Salty.
Onion: Sweet.
Mushrooms: Umami.
Spinach: Bitter and slightly sweet.


The other ingredients are mostly just adding flavor and texture, which may not be necessary in your case.  Also, keep in mind that the oil, feta, and tortillas (assuming you are using flour tortillas) all contain fat, so you may want to bump-up the sourness a bit.  Also, if you found that the spinach was too bitter, you could offset it by reducing the amount and perhaps caramelizing your onions a bit more to add sweetness.  You could also add some balsamic vinegar to your onions as they caramelize which will both make them sweeter and also add some acidity.

As a final suggestion, make sure to write everything down!  That way, then next time you try this recipe you will know (1) how much of each ingredient you used before, and (2) what ingredients you'd like to alter.