Prepare the Mushrooms: Preheat your oven to 375°F (190°C). Clean the portabella mushrooms by gently wiping them with a damp cloth and remove the stems and gills with a spoon. Brush the caps lightly with olive oil and set aside.
Cook the Filling: Heat 1 tablespoon of olive oil in a skillet over medium heat. Sauté the garlic for 1-2 minutes until fragrant. Add the spinach and cook until wilted. Remove from heat and let cool slightly.
Mix the Filling: In a mixing bowl, combine the cooked spinach, cream cheese, Parmesan cheese, thyme, salt, and black pepper. Mix until well combined.
Stuff the Mushrooms: Spoon the filling evenly into the prepared mushroom caps, mounding slightly. Sprinkle breadcrumbs on top if desired for extra texture.
Bake: Arrange the stuffed mushrooms on a baking sheet and bake for 20 minutes or until the mushrooms are tender and the filling is golden and bubbly.
Serve: Let the mushrooms cool for a few minutes before serving. Garnish with a sprinkle of fresh parsley or extra Parmesan if desired.