Heat the oil in a large saute pan on medium heat.
Add the onions and mushrooms, season with salt and pepper and cook until lightly browned, stirring occasionally. (FYI - you can’t over these until they’re burned)
Add the garlic and cook for 2-3 more minutes.
Optional - add vinegar or lemon the last minute.
Add half of the spinach and stir until wilted then add the second half and sauteed until wilted, about 2 minutes.
Enjoy right away or cool down and use it for meal prep!