Preheat your oven to 425°F (220°C) and prepare a baking sheet with nonstick cooking spray.

Heat 2 tablespoons of extra virgin olive oil in a skillet over medium-high heat. Add diced onion and sauté for about 5 minutes until golden brown.

Add sliced mushrooms to the skillet. Cook for 8 to 10 minutes, stirring frequently until browned and moisture evaporates.

Lower the heat to medium, add minced garlic and cook for 30 seconds until fragrant.

Season the mushroom mixture with kosher salt and black pepper to taste. Set aside.

In a medium bowl, combine ricotta cheese with 1 tablespoon of chives. Season with salt and pepper, mixing until smooth.

Lightly brush one side of each slice of rustic bread with olive oil and place them oil-side down on the baking sheet.

Spread an even layer of the ricotta mixture over each slice of bread.

Spoon the sautéed mushroom mixture on top of each ricotta-covered bread slice evenly.

Sprinkle shredded Gruyere cheese over the top of each sandwich.

Bake in the preheated oven for about 15 minutes or until the toppings are hot and bread's underside is golden brown.

Remove from oven, sprinkle remaining chives over the top, and serve immediately.