Preheat your oven to 375°F (190°C).
Clean the portobello mushrooms with a damp cloth and remove the stems.
Brush both sides of the mushrooms with olive oil.
In a bowl, mix the ricotta cheese, spinach, cherry tomatoes, and walnuts.
Season the mixture with salt and pepper to taste.
Stuff each portobello mushroom cap generously with the cheese mixture.
Place the stuffed mushrooms on a baking sheet lined with parchment paper.
Sprinkle grated parmesan cheese over the top of each stuffed mushroom.
Bake in the preheated oven for 25-30 minutes until the mushrooms are tender.
Garnish with fresh herbs before serving.