Start by heating butter and olive oil in a large skillet over medium-high heat. Toss in the sliced mushrooms along with a sprinkle of salt and pepper. Stir them well to coat, and cook for about 8-10 minutes until the mushrooms shrink and turn rich brown. Add Worcestershire sauce and give it another quick stir. Set the mushrooms aside to cool.

Combine mayonnaise, sour cream, and melted butter in a bowl. Mix until smooth. Then, stir in minced garlic, onion flakes, black pepper, and all the cheeses. Fold in the ingredients gently so they’re evenly mixed. You should end up with a thick, dreamy spread.

Preheat your broiler to high. Arrange sourdough slices on a baking sheet. Toast one side until golden, then flip them to toast the other side. This ensures your bread stays crispy under the rich toppings.

Spread the creamy cheese mixture thickly onto each bread slice. Lay your reserved mushrooms decoratively on top. Place the toasts under the broiler and watch closely as the cheese bubbles and browns. This should take about 3-5 minutes. Garnish with freshly chopped green onions before serving.

Serve the toasts hot and watch as everyone grabs seconds. Trust me, they’re irresistible!