Cut broccoli florets into bite-sized pieces. Peel and chop stalks., In a large saucepan, heat oil over medium-high heat; saute mushrooms until tender, 4-6 minutes. Stir in soy sauce; remove from pan., In same pan, combine broccoli stalks, carrots, celery, onion, garlic, broth and water; bring to a boil. Reduce heat; simmer, uncovered, until vegetables are softened, 25-30 minutes., Puree soup using an immersion blender. Or cool slightly and puree soup in a blender; return to pan. Stir in florets and mushrooms; bring to a boil. Reduce heat to medium; cook until broccoli is tender, 8-10 minutes, stirring occasionally. Stir in lemon juice.