In a large pot with a lid, melt 2 tbsp. butter over medium-high. Add the chicken and rosemary; season with salt and pepper. Cook, stirring and breaking up the chicken with a spoon, until browned, about 5 minutes. Transfer to a bowl.
In the pot, melt the remaining 2 tbsp. butter over medium. Add the squash, celery root, leek, celery seed and half the thyme; season. Cook, stirring occasionally, about 2 minutes, until the vegetables are coated with butter.
Stir in the stock; cover and bring to a boil. Stir in the pasta and chicken and cook, uncovered, over medium, stirring occasionally, until the pasta is al dente and the vegetables are tender, about 6 minutes. Divide among 4 bowls. Top with the crème fraîche, lemon zest and remaining thyme.
Want more chicken soup? Try these recipes!
- Chicken & Lime Tortilla Soup
- New England Chicken Chowder
- Chicken & Red Lentil Soup
Rachael Ray Every Day's January/February 2017 cover