Place six cups of water in a medium saucepan over high heat.
Add the sweet potatoes, and when the water boils, reduce the heat to a simmer. Cook for about 15 minutes.
While the potatoes are cooking, place two cups of the water in a small saucepan over high heat.
When the water boils, add the dried mushrooms and remove from the heat. Let stand for about 10 minutes and strain the mushrooms out of the broth into a small bowl. Set the mushrooms aside.
(If you are using fresh mushrooms: Place 1 tsp. olive oil in a small saucepan over medium heat. Saute the mushroom slices gently for about ten minutes. Add the 2 cups of water to the saucepan, bring to a boil, then remove from heat and allow to stand for 10 minutes. Strain the mushrooms out of the broth, set the mushrooms aside, and continue as below. )
Return the broth to the sauce pan and add the marjoram, rosemary, thyme, and salt.
Place the broth over low heat and reduce until there is one cup broth. Set aside.
While the broth is reducing, place the olive oil in a large skillet over medium high heat.
Add the onions and cook for about 3 minutes. Stir frequently.
Add the celery and carrots and cook for about 3 minutes. Stir frequently.
Remove the mixture to a small bowl to cool.
When the potatoes are tender, drain them and place the potatoes in a small bowl with the butter.
Mash the potatoes with a whisk until the potatoes are smooth.
Add the buttermilk and whisk until smooth.
When the onion and carrot mixture is cool, add the grated cheddar cheese and toss to blend well.
Preheat the oven to 325°F.
Coarsely chop the mushrooms.
Place the cornstarch in a small bowl and add 4 tablespoons of cold water. Stir well.
Place the mushroom broth over medium high heat. When hot – but not boiling – whisk in the cornstarch mixture and remove from the heat. Whisk constantly until the sauce thickens.
Add the mushrooms to the thickened sauce, stir, and divide between two ovenproof bowls.
Top with the onion and cheddar mixture.
Top with the mashed sweet potatoes, spray lightly with olive oil, and place in the oven.
Bake for 20 minutes.
Serve.