In roasting pan, toss mushrooms and onion with 2 tbsp (25 mL) of the oil. Roast in 400°F (200°C) oven, stirring once, until tender, about 35 minutes.
In large saucepan, heat remaining oil over medium-high heat; fry celery and garlic, stirring, until softened, about 3 minutes. Stir in broth, 2 cups (500 mL) water, the mushroom mixture and any juices, and potato; bring to boil. Reduce heat, cover and simmer until vegetables are tender, about 10 minutes.
Stir in balsamic vinegar, soy sauce and pepper. Using immersion blender, purée soup until smooth. (Alternately, let soup cool slightly. In batches, purée in blender until smooth. Reheat in clean pot.)
In bowl, whisk cream until soft peaks form. Ladle soup into heatproof sundae glasses; spoon whipped cream on top, and garnish with cherry tomato on cocktail stick.