Position oven rack 6" from broiler. Preheat oven to 550° or as high as your oven will go. Add the baking steel on the rack under the broiler. Heat baking steel 60 minutes. Position a baking sheet on the lower rack (bottom half of the oven).
If using a pizza peel, flour it generously with semolina flour. Keep extra flour handy to re-flour the peel as necessary for each pizza. Position your pizza base on the peel and shake periodically as you prep the toppings to ensure the pizza slides and stays loose.
If using parchment paper instead of a pizza peel, cut paper to the correct size with a couple of inches excess around the edges of the pizza for each base. Position a pizza base on the parchment paper. Proceed with toppings.
Mix the tomato, oregano and salt together. Divide evenly over the 4 pizza bases to within 1" of the outer edge.
Chop bacon into ½" pieces. Microwave on medium high about 2 minutes.
Divide the cheese into 4 equal portions and distribute evenly over the pizza bases.
Divide onion, olive, bacon and mushrooms into 4 portions and spread evenly over bases.
Turn the oven to Broil setting. Carefully transfer the prepared pizza onto the baking steel.
Broil about 2 minutes. Top will have some char and bottom should be dry and firm. Using a large spatula or thin edged pizza peel transfer the pizza to the baking sheet in the lower portion of your oven. See Note 3. Turn oven to 500 ° and allow pizza to remain on baking sheet another 2-3 minutes. See Note 4 if cooking more than 1 pizza immediately.
Remove pizza from oven and garnish with fresh parmesan and basil. Allow to sit a couple of minutes before slicing in to quarters.