Finely dice the yellow onion, mince the garlic, and slice the cremini mushrooms.
Heat the extra virgin olive oil in a large skillet over medium-high heat.
Add the ground beef to the skillet, breaking it apart with a spatula, and cook until browned and fully cooked through.
Remove the beef from the skillet and set aside on a plate, leaving any rendered fat in the pan.
Add the onions and mushrooms to the skillet, sautéing until the mushrooms are golden brown and tender.
Stir in the minced garlic and cook for 1 minute until fragrant.
Return the beef to the skillet and add the tamari, sea salt, and black pepper, stirring to combine all flavors.
Add the baby spinach and toss gently for 1-2 minutes until just wilted.
Serve immediately while hot for a clean and protein-packed meal.