Preheat oven to 400°F. In sauce pan, combine 8 tablespoons butter and flour and cook 5 minutes over medium heat. Add milk and heavy cream, bring to a simmer for 10 minutes.
Turn heat down to low and slowly add in mascarpone and fontina while whisking until all cheese is melted. Remove from heat and season with nutmeg, cayenne, salt and pepper. In large sauté pan over high heat, add oil, and sauté mushrooms until tender.
To mushrooms, add thyme, garlic, shallot and remaining 2 tablespoons butter, cook for another minute, then season with salt and pepper.
In large bowl, add cooked pasta, cheese sauce, mushroom mixture, chives, green onions, truffle oil, lemon juice, and check for seasoning.
Place mixture into baking pan and top with grated morel & leek jack cheese and white cheddar. Bake until cheese is melted and golden brown, about 15 minutes.