Preheat the oven to 400ºF and set a wire rack on a baking sheet. Place the mushrooms open-side up on the wire rack. Drizzle the mushrooms with olive oil and season with salt and pepper. Use your hands to massage the oil and seasonings into the mushrooms.
Place the mushrooms into the oven and bake them for 12-15 minutes or until they begin to soften and release moisture.
Remove the mushrooms from the oven and let them rest while you prepare the filling.
Heat a large skillet over medium/high heat and add olive oil to the skillet. When the olive oil is fragrant add the onion to the skillet with a pinch of salt and sauté the onion for 2-3 minutes. Add the garlic and sauté for an additional minute.
Add the artichoke to the skillet and season with salt and pepper. Toss all the ingredients together and sauté for 1-2 minutes.
Turn the heat to medium heat and add the fresh spinach to the skillet and toss. Let the spinach cook down and wilt for about 1-2 more minutes.
Next, add the cream cheese to the skillet and use a spatula to stir all of the ingredients together until the cream cheese melts and the whole mixture becomes thick and creamy.
Add about ¼ cup of the spinach artichoke mixture to the well of each portobello mushroom. The mixture should be a mound of filling. If your mushrooms don’t have a well in the middle, that’s okay! You can still scoop the filling on top of the mushroom.
Turn the heat on the oven to broil.
Sprinkle mozzarella cheese over the top of each mushroom and place the mushroom snack into the oven for 2-3 minutes or until the cheese has melted and begins to brown. Check your mushrooms after 1 minute as each oven is different.
Remove the mushrooms from the oven and enjoy.