Step 1

Melt butter in a medium saucepan over medium-high heat. Add leeks and garlic and cook, stirring occasionally, until translucent, about 2 minutes. Stir in mustard, then flour; cook until mixture has a slightly nutty aroma, about 1 minute.

Step 2

Gradually whisk in broth until combined. Add broccoli stems and season with salt and pepper; increase heat to high and bring to a boil, whisking constantly. Add broccoli florets; reduce heat to low and simmer partially covered until broccoli is tender, about 8 minutes. Stir in spinach and cook 1 minute more; remove from heat.

Step 3

Working in batches, purée soup in a blender. Return to pot and reheat over medium-low. Whisk crème fraîche with 1 tablespoon water in a small bowl. Serve soup, drizzled with crème-fraîche mixture and garnished with pretzels.