In a 2-quart saucepan or soup pot, saute onion in oil for 5 minutes until limp. Add vegetables and apple and stir until coated. Add water and bouillon cube (or broth), bring to a boil, reduce heat, cover, and simmer for 35 minutes or until vegetables are very tender when pressed with the back of a spoon. Puree with an immersion blender or in batches in a regular blender. Mix mascarpone with milk or broth until thinned enough to pour; and transfer to a squeeze bottle. Ladle soup into cups or bowls and squirt mascarpone on top of each serving.