Preheat the oven to 375°F (190°C). In an oven-proof dish, toss the mushrooms with 4 Tbsp. (60 mL) olive oil and the salt. Arrange the mushrooms in a fairly flat layer, transfer to oven, and roast for 30 minutes, stirring once or twice. Toss in the garlic and thyme and continue cooking for another 15 minutes.
Meanwhile, put the remaining 2 tsp. (10 mL) olive oil and chopped onion in a small frying pan over medium heat. Sauté the onion until light golden. Remove onion and set aside. In the same pan, sauté the pancetta until it has rendered almost all of its fat. (You may need a bit more oil to get started.) Deglaze the pan by adding 2 Tbsp. (30 mL) Madeira and scraping up any browned bits from the bottom of the pan.
In a soup pot, bring the 4 cups (950 mL) stock to a simmer over medium-high heat. Add the mushrooms and onions, as well as the pancetta and deglazed liquid from the pan. Now is the time to decide if you want a textured or smooth soup. Using a hand blender or in a blender or food processor purée accordingly. Mix in the remaining 1 Tbsp. (15 mL) Madeira and the cream. If the soup is too thick, stir in the extra ½ cup (120 mL) hot stock. Season with freshly ground black pepper.
Serve warm, decorated with 2 or 3 long stalks of chives per bowl.