Preheat the grill or broiler. Wrap the garlic in aluminum foil and place on the grill rack or under the broiler about 8 inches from heat source. Grill or broil the garlic for about 20 minutes.

In a medium bowl, toss the onions with 3 tablespoons of olive oil. Brush the remaining oil generously over the portobellos. Place on a shallow pan and grill-smoke over medium-hot part of barbecue with the lid closed or broil about 8 inches from heat source. Either way, the mushrooms and onions will take about 12 - 15 minutes and should be slightly charred.

Remove, cool slightly and chop the onions and mushrooms and keep separate.

Remove the garlic. Open the aluminum foil and let the garlic cool until it is cool enough to handle.

In a large pot over medium heat 2 more tablespoons of olive oil. Add the chopped charred onions and cook for five minutes. Add the mushrooms and roasted garlic and toss to coat. Sprinkle with salt, pepper, tarragon and marjoram. Sprinkle in the flour and turn until the mixture becomes a little pasty. Pour in the wine. As soon as the alcohol steams off add the stock. Bring to a boil over high heat, reduce to medium, and simmer for 15 minutes, for the flavors to meld. Remove from heat. With an immersion blender, puree the soup in the pot. Alternatively, let it cool slightly, drain it, reserve the liquid and puree the solids to a creamy consistency in the food processor at high speed. Place back in the pot and stir over medium heat. Add the balsamic, stir well, adjust seasoning with salt and pepper and serve garnished with some of the nuts.