Heat a large skillet over medium-high heat. Add 1 Tbsp avocado oil and heat until shimmering.

Add the diced onion to the skillet and sprinkle with 1/2 tsp sea salt. Cover and cook, stirring occasionally, until the onion softens and begins to brown (near-caramelized), about 8–10 minutes.

Stir in the chopped mushrooms and minced garlic. Cook, stirring occasionally and scraping up any browned bits, until the mushrooms soften and turn brown, about 5 minutes.

Push the cooked vegetables to one side of the skillet. Add the 1 lb ground beef to the empty side and let it brown undisturbed for 2–3 minutes. Use a spatula to break the beef into smaller pieces, then continue cooking and stirring until the beef is cooked through with no pink remaining, about 4–6 more minutes.

Stir the vegetables and beef together. Add 2 tsp dried oregano and, if using, 2 Tbsp coconut aminos or soy sauce. Add the 5 ounces (about 2 big handfuls) baby spinach and stir to combine.

Cover the skillet and cook until the spinach is wilted, about 2–3 minutes. Uncover and stir.

Taste and adjust seasoning as needed — add more sea salt or a splash of coconut aminos/soy sauce to taste. Remove from heat and serve immediately.