Step 1

In a large saucepan, melt 4 tablespoons of the butter. Add the leeks, onions, garlic and a generous pinch each of salt and pepper and cook over moderately high heat, stirring, until softened but not browned, about 12 minutes. Add the celery and cook, stirring, until just starting to soften, about 3 minutes. Add 8 cups of water and bring to a boil, then reduce the heat to moderate. Simmer, stirring occasionally, until the vegetables are very tender, 35 to 40 minutes.

Step 2

Meanwhile, in a large skillet, cook the bacon over moderate heat, stirring occasionally, until browned and crisp, 8 minutes. Using a slotted spoon, transfer the bacon to half of a paper towel–lined baking sheet to drain. Melt the remaining 2 tablespoons of butter in the bacon fat. Add the bread and cook over moderately high heat, stirring, until browned and crisp, 8 minutes. Transfer to the other side of the prepared baking sheet; season with salt and pepper.

Step 3

Working in batches, puree the soup in a blender until smooth. Return the soup to the saucepan, whisk in the cre?me frai?che and season with salt and pepper. Serve hot, topped with the bacon, croutons and a drizzle of lemon olive oil.