Preheat oven to 400 degrees.
Clean the mushrooms, including the stem by wiping them with a paper towel.
Remove the stems from the mushrooms and dice them into small pieces and set aside.
In a skillet, over medium heat, melt the unsalted butter and add the minced garlic, diced mushroom stems, and the baby spinach. Saute for about 5 minutes to cook lightly brown the garlic and mushroom stems and to wilt the spinach.
Remove the pan from the heat. Add the diced artichokes, cream cheese, shredded mozzarella, and panko breadcrumbs. Season with salt and pepper to taste.
Place the olive oil in a small bowl, use a pastry brush to dip into the olive oil and coat the mushroom caps lightly with the olive oil. Then place the mushrooms on a cookie sheet.
Use a teaspoon to fill each mushroom cap, press filling inside the cavity of the mushroom. Usually, about 2 teaspoons of the filling will fill the mushroom.
When all mushrooms are filled, place the cookie sheet in the oven for about 15 minutes until mushrooms are golden brown and cheese is melted.