Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Melt 1 tablespoon of unsalted butter in a non-stick skillet over medium heat. Add diced onion and sauté until translucent, about 5 to 7 minutes. Add pressed garlic and sauté for another minute.

Increase heat to medium-high and add another tablespoon of butter. Add white mushrooms and sauté until their juices evaporate, about 10 to 12 minutes.

Cook bacon in the same skillet over medium heat until crispy, about 7 minutes. Drain excess fat, chop, and add to the mushroom mixture.

In a mixing bowl, combine all sautéed ingredients, including cheese, parsley, mayonnaise, salt, and pepper. Mix well.

Slice baguettes into 1/2-inch pieces and spoon the filling generously onto each slice.

Place the filled baguette slices on the baking sheet and bake for about 15 minutes until cheese is melted and bubbly.

Garnish with fresh parsley and serve warm.