Heat a tablespoon of butter in a skillet over medium heat until it melts and starts to bubble gently.
Add the sliced mushrooms to the skillet and sauté for about 5-7 minutes, stirring occasionally, until they are tender and have turned a light golden brown.
Stir in the minced garlic and cook for another 30 seconds to 1 minute, until fragrant and slightly softened.
Add the remaining butter to the skillet, allowing it to melt, and then pour it over the mushrooms and garlic mixture.
Gradually add the fresh spinach a handful at a time, stirring gently until each addition wilts and reduces in volume, about 2-3 minutes total.
Cook everything together for another minute or two to meld the flavors, then remove from heat.
Transfer to a serving dish and enjoy warm as a side or over grains for a quick, nourishing meal.