Wash and remove the stem from the portobellos. Season them front and back with salt and pepper and drizzle both sides with olive oil.
On medium-high heat, heat a couple of large skillets with about a tablespoon of olive oil. Cook portobellos for about 5-7 minutes on each side or until they are well cooked and get dark around the edges. You can also cook these on the grill which works very well because of how big they are. Make sure not to undercook them. When they are finished cooking place them gill side up on a baking sheet.
While the portobello mushrooms are cooking dice up the onion and heat a couple of tablespoons of olive oil in a large skillet. Cook the onion on medium heat until translucent and fragrant. Season the onion with about a ¼ teaspoon salt and pepper.
Grate or press in all four large cloves of garlic. Stir and cook for just a couple of minutes.
Add in spinach leaves. Season the spinach leaves with another ½ teaspoon of salt and pepper and a drizzle of olive oil. Stir and cook until spinach leaves are wilted and cooked down. Taste and add salt or pepper if necessary.
Evenly distribute the spinach mixture into the portobellos.
Top each portobello evenly with mozzarella cheese.
Turn the broiler on high on your oven.
Place the stuffed portobellos that are on a cookie sheet under the broiler for about 5 minutes or until cheese is melted and slightly browned.
Dice the tomato and sprinkle lightly with salt and pepper. Roll the fresh basil and gently cut in stripes.
Remove portobellos from the oven and top with tomato and basil.