Clean the portobello mushrooms with a damp cloth and remove the stems, gently scooping out the gills with a spoon to create a cavity for stuffing.
In a medium skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant.
Add the chopped spinach to the skillet and cook until wilted, about 2-3 minutes. Then, stir in the artichoke hearts, cooking for another 2 minutes. Remove the skillet from heat and let cool slightly.
In a large bowl, combine the softened cream cheese, Parmesan cheese, mozzarella cheese, lemon juice, red pepper flakes (if using), salt, and pepper. Mix until well combined.
Fold in the spinach and artichoke mixture into the cheese mixture until fully incorporated.
Spoon the cheese and vegetable mixture generously into each of the portobello mushroom caps, pressing down slightly to pack it in.
Arrange the stuffed mushrooms on a baking tray lined with parchment paper.
Bake for 25-30 minutes or until the mushrooms are tender and the tops are golden brown.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.