Preheat your oven to 400 degrees. Place the olive oil, tomatoes, leeks, salt, and pepper in a medium-sized bowl and toss to coat. Place the tomatoes in a large baking dish and roast for 1 hour, stirring every 20 minutes. At the 40 minute mark, pop in the leeks. 

Melt 2 tbsp of butter in a large saucepan over medium heat. Then add the rosemary and stir for 1 minute or until you can smell it. Add the broth and the roasted vegetables. Simmer for 20 minutes, stirring occasionally. Then, blend the ingredients using a heat-proof blender or an emersion blender.

Melt the remaining butter in a small frying pan over low heat and add the flour. Whisk continuously to create a smooth roux. Add the roux to the soup along with some salt and pepper. Stir the soup thoroughly and let it simmer for 3 more minutes.