Preheat the oven to 400 degrees F.

Wash and pat dry the portobello mushrooms. Using a spoon, scoop out the gills and stems. Lightly coat each cap with olive oil, sprinkle with salt and pepper, and place on a foil-lined baking sheet. Roast for 10 minutes on one side, flip, then roast an additional 5 minutes. Remove from the oven and decrease oven heat to 375 degrees F.

Pour 1/2 cup of the tomato sauce in an oven-safe skillet and evenly spread. Transfer the mushrooms to the skillet, gill-side up.

Combine 1-1/2 cups of tomato sauce, the cooked rice, and the baby spinach in a large pot. Heat over medium and stir until spinach has wilted. Taste rice for flavor and add sea salt as desired.

Scoop the rice mixture into the mushroom caps and top with the remaining tomato sauce and mozzarella cheese. Bake for 12 to 15 minutes, until cheese is melted and browned. For crispy cheese, broil the skillet for 2 to 3 minutes under the broiler.

Serve with freshly grated parmesan and basil.