Heat the oil and butter in Dutch oven pot over medium heat. Add the leeks, onion, carrot and celery. Reduce heat to medium-low, and sauté, stirring frequently, until translucent and very tender, about 5 minutes. Add garlic & broccoli & sauté together lightly.
Stir in the stock and heavy cream, add salt and pepper. Raise heat to medium-high, cook for 4 minutes
Cook over high heat, stirring occasionally, 4 to 5 minutes.
Use an immersion blender to puree the soup (if using a blender, make sure soup is not too hot; remove middle part of lid and cover with towel to avoid splattering).
Add cheese and heat until melted. Add lemon zest and lemon juice, and adjust seasoning.
Garnish with a sprinkle of cheese. Serve with piece of crusty bread.