Melt the butter in a large heavy-based saucepan over medium heat. Reduce the heat, then add the mushrooms and leeks and cook, stirring, until the vegetables have softened.
Increase the heat, add the flour and cook, stirring, for 1 minute. Remove the saucepan from the heat and gradually stir in the stock. Return to the heat and stir until the soup boils and thickens. Reduce the heat and simmer for 5 minutes. Stir in the cream and parsley, season to taste with salt and pepper, and gently heat through for a few minutes, taking care not to let the soup boil.
Meanwhile, heat the oil in a frying pan until sizzling. Add the bacon and cook, stirring, until crisp. Drain on paper towels.
Ladle the soup into four warmed serving bowls and top each with a spoonful of sour cream. Sprinkle the bacon over the top and garnish with sage leaves.