Heat 1 tablespoon avocado oil in a large skillet over medium-high heat.
Add the diced yellow onion to the skillet, sprinkle with a pinch of sea salt, cover, and cook, stirring occasionally, until the onion begins to brown and near-caramelize, about 8 to 10 minutes.
Stir in the chopped mushrooms and minced garlic and cook, stirring, until the mushrooms soften and brown, about 5 minutes.
Push the vegetables to the side of the skillet and add the ground beef; let it brown for 2 to 3 minutes, then break it up with a spatula and continue cooking until no pink remains.
Stir the beef and vegetables together, then add 2 teaspoons dried oregano and 2 tablespoons coconut aminos or soy sauce if using; mix well.
Add the baby spinach and 1/2 teaspoon sea salt, stir to combine, cover, and cook until the spinach wilts, about 3 minutes.
Taste and adjust seasoning, adding more sea salt or a splash of soy sauce or liquid aminos if desired, then serve.