Coat a large saucepan with cooking spray and set pan over medium heat. Add leeks and sauté until soft, about 5 minutes. Add tomatoes, thyme and bay leaf; cook until tomatoes break down, about 5 minutes.
Add broth and bring to a simmer; reduce heat to low, cover and simmer 10 minutes. Remove from heat and remove bay leaf.
Puree tomato mixture in saucepan using an immersion blender. Or, working in batches, puree tomato mixture in a blender until smooth (be careful not to splatter hot liquid); return puree to saucepan and set pan over low heat.
Add milk and simmer 1 minute to heat through. Remove from heat and stir in basil; season to taste with salt and pepper. Garnish with thyme and serve.