Creamy Broccoli-and-Spinach Soup

Ingredients

3 tablespoons unsalted butter
2 leeks (about 1 pound), white and light-green parts only, halved lengthwise and thinly sliced crosswise, well washed, and drained (2 cups)
3 cloves garlic, smashed and peeled
2 teaspoons Dijon mustard
3 tablespoons unbleached all-purpose flour
4 cups low-sodium chicken or vegetable broth
1 small head broccoli (about 12 ounces), stems peeled and cut into ¼-inch coins, head cut into bite-size florets
Kosher salt and freshly ground pepper
5 ounces baby spinach, stems removed (6 packed cups)
0.333 cup crème fraîche
Store bought pretzel nuggets, for serving (optional)

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.
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.
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.