Heat oven to 400°F.
Roasting the garlic: Cut about 1 inch off the top of both heads of garlic, brush each garlic bulb with about a teaspoon of olive oil.
Then sprinkle with salt and pepper.
Place in a small baking dish or on a cookie sheet.
Cover with foil, and roast about 45 minutes.
Garlic should be soft.
Allow garlic to cool.
Squeeze bulbs to release the roasted garlic.
Set roasted garlic aside.
Making the vegetable stock: Heat 1-2 tablespoons olive oil in large saucepan.
Add half the onions and cook over low heat, allowing the onions to caramelize—about 25 minutes.
Add the mushroom stems, half the carrots, and the celery and continue cooking until vegetables soften—about 10 minutes.
Add 8 cups water, parsley sprigs, and bay leaf.
Increase heat to high, and bring stock to a boil.
Then reduce heat to low and simmer for 45 minutes.
If necessary, add enough water to bring stock to 6 cups.
Remove bay leaves and parsley springs.
Set aside.
To make the soup: Heat 1-2 tablespoons olive oil in a large Dutch oven over medium heat.
Add about one third of the mushrooms and sauté about 3 minutes.
Add the next third of the mushrooms with another tablespoon or so of olive oil and repeat, sautéing another 3 minutes.
Add the final third of mushrooms and add additional olive oil only if necessary.
Sauté another 3 minutes.
Add the remaining onion and cook until softened—about 3 minutes.
Add the butternut squash and remaining carrots and cook for 2 minutes.
Add the vegetable broth, salt, pepper, roasted garlic, and all the homemade vegetable stock.
Stir to combine, and bring to a boil.
Reduce heat to low and simmer until vegetables are tender—about 45-60 minutes.
Stir in chopped parsley and lemon juice and serve immediately.