Preheat the Traeger Grill to 375°F and line a sheet pan with parchment paper.

Divide the prepared mushrooms into 2 parts, about 2 1/2 pounds for the soup and about 1/2 pound for the topping.

Place the topping mushrooms on the sheet pan, then coat with olive oil and add thyme leaves, salt, and pepper.

Place the topping mushrooms on the preheated Traeger Grill and cook until browned and crispy.

Heat a large pot over medium heat, then pour in olive oil.

Add the red onion, season with salt, and cook until softened and translucent, about 5–7 minutes.

Add the soup mushrooms and cook until starting to caramelize, about 10–12 minutes.

Add the rosemary, thyme, and garlic, then cook for 2 minutes while stirring.

Add the balsamic vinegar.

Add the chicken stock and bring to a simmer.

Cook for 5 minutes, then transfer to a blender and blend until smooth.

Pour the soup back into the pot, then add the heavy cream, butter, Parmesan cheese, lemon zest, salt, and pepper.

Place the pot back over medium-low heat and let the soup gently blip away for 10 minutes.

Cut the loaf of bread in half, slather on the garlic butter, and top with Gruyere cheese and thyme leaves.

Place the bread on the Traeger Grill and cook until melted and golden, then cut into strips.

Divide the soup into bowls, top with crispy mushrooms, drizzle with olive oil and heavy cream, and serve with cheesy bread.