Toss all of the salsa ingredients together right in your serving dish, and season to taste with the salt. Set the salsa aside until you're ready to serve.
Now Saute the Spinach and Onions
Heat the oil to medium-high in a large frying pan, and add the garlic and onion. Saute, stirring occasionally, until the onion softens and begins to brown ever so slightly, about 5 minutes.
Add the spinach, and cook, stirring, until the spinach softens and begins to wilt. Add the black beans, stirring to combine well, and cook until the spinach fully wilts and turns bright green. Season with salt and pepper.
Make the Tacos
Divide the spinach-bean mixture between your tortillas. Top each tortilla off with pineapple salsa and a slice of avocado
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Notes
About the salsa: It's key that you make the salsa first, because that will allow some time for the flavors to meld while you prepare the rest of the taco ingredients. You can make the salsa up to two days ahead, if you prefer.
About cooking the spinach: You don't want the spinach to be fully cooked when you add the beans. Add the beans when the spinach had reduced in volume by about ½. It will reduce even more during the total cooking process.