Preheat oven to 400 degrees Fahrenheit. Choose a large cast iron skillet (see link below) or baking pan that will fit the mushrooms.
Wipe each mushroom with a wet dishrag or paper towel, to clean. Turn each mushroom over and pop out the big stem, pushing with your thumb to one side. Reserve the stem. Use a spoon to remove the gills in the underbelly, raking it along; they will scrape out easily. Reserve them.
Chop the stem into small pieces. Heat fat in medium size pan. Add chopped stem and sea salt. Saute over medium-high heat for 8-10 minutes. Add gills, garlic and dried basil, reducing heat to medium-low. Saute for 2 additional minutes.
Transfer sauteed mushroom to medium size bowl. Add tomato paste and stir well.
Fill the cavity of each mushroom with tomato sauce. Place mushrooms into baking pan. Top with remaining preferred ingredients: grated cheese, meat of choice, olives. Reserve fresh tomato and fresh basil to put on after the pizzas are baked.
Bake 25 minutes, or until cheese is well melted and pizzas looks sizzling and a bit crispy around the edges.
Top with optional fresh tomato slices and fresh herbs.