In a large skillet, over medium-high heat, add the olive oil and butter. Once the butter is melted, add the mushrooms, salt, and ground black pepper. Stir to coat the mushrooms in the oil and butter mixture. Cook mushrooms for five minutes.
Add the Worcestershire sauce to the mushrooms, stir, and cook for another five minutes, or until mushrooms are a deep brown color. Once cooked, remove from skillet and transfer to a plate to cool. Remove the better-looking mushrooms, leaving behind the broken or smaller pieces. Set the reserved mushrooms aside.
To make the cheese spread, add the mayonnaise, sour cream, and melted butter to the bowl first. Whisk those together until combined. Next, add in the garlic, onion flakes, and black pepper. Stir those into the mayonnaise and sour cream mixture. Finally, add the four cheeses and the green onions. Fold those into the cheese until evenly dispersed throughout the mayo and sour cream. Set aside.
Place the bread in a single layer onto a large sheet pan. Put it under the broiler until lightly browned. Remove the bread from the oven and flip them over. Toast the second side as well.
Now, allow the bread to cool slightly so that the slices are just warm to the touch.
Fold the cooled mushrooms into the cheese mixture. Not the reserved mushrooms! Spread the cheese mixture onto each toast.
Top the toasts with the remaining reserved mushroom slices. Place the sheet pan back in the oven and carefully watch while the cheese begins to bubble and brown. Don’t burn the toasts, but keep them under the broiler until you are satisfied with the color. The cheeses should be melted all the way through. This will take about 3-5 minutes under a 500 degrees F broiler.
Top with more green onions as a garnish and serve immediately.