Preheat the oven to 425°F (220°C).

Prepare the shiitake mushrooms by discarding the stems and slicing the caps 1/2 inch thick.

Cut the red onions into 1/2-inch wedges.

On a large rimmed baking sheet, combine the sliced mushrooms and onion wedges.

Drizzle 5 tablespoons of extra-virgin olive oil over the mushrooms and onions.

Season generously with kosher salt and freshly ground pepper to taste.

Toss the vegetables to ensure they are evenly coated with oil and seasonings.

Roast in the preheated oven for approximately 30 minutes, tossing once halfway through, until the mushrooms and onions are tender and slightly caramelized.

Remove the baking sheet from the oven.

Sprinkle 1/2 teaspoon of thyme leaves over the roasted vegetables and toss to distribute evenly.

Distribute 3 ounces of Gorgonzola Dolce, cut into 1/2-inch pieces, over the vegetables.

Return the baking sheet to the oven and roast for an additional 5 minutes, or until the Gorgonzola cheese is melted and slightly browned.

Remove from the oven and let cool slightly.

Transfer the roasted mushrooms and onions to plates and serve immediately.