Preheat Oven to 350˚F and line large baking sheet with parchment paper. Place a large non-stick skillet over medium heat. Add 1 Tbsp butter and chopped onion. Saute stirring often until soft and lightly golden (5-7 min). Add garlic and stir until fragrant (1 min). Transfer to mixing bowl.

In the same skillet over medium/high heat, add 1 Tbsp butter and sautee mushrooms until soft, juices are released and evaporated (10-12 min). Transfer mushrooms to the same mixing bowl.

Add bacon to the same skillet and sauté until browned (7 min). Transfer to paper towel lined plate to drain fat before adding to the mixing bowl.

Add 2 1/2 cups shredded cheese, 1/4 cup chopped parsley, 2 Tbsp mayonnaise, 1/4 to 1/2 tsp salt, and 1/4 tsp pepper, or season to taste. Stir to combine.

Spoon generously onto toasts, using spoon to mound and shape the mixture. Bake at 350˚F for 15 min. Garnish with more fresh parsley if desired and serve warm.