2 tablespoons unsalted butter
1 onion, diced
1 stalk celery, diced
Kosher salt
freshly ground pepper
2 teaspoons chopped fresh thyme
2 garlic cloves, minced
2 tablespoons all-purpose flour
3 cups vegetable broth
1 9-ounce package frozen artichoke hearts, thawed
1 bay leaf
Zest of 1/2 lemon
3 ounces baby spinach
1 cup fresh parsley leaves
½ cup sour cream
Melt the butter in a Dutch oven or stockpot set over medium heat. Add the onions and cook, stirring frequently, until softened, about 5 minutes. Add the celery, season with salt and pepper, and cook until the celery is softened, about 5 minutes more. Stir in the thyme, garlic, and flour and cook, stirring, for 2 more minutes, until the vegetables are coated with the flour.
Add the broth, artichokes, bay leaf, and lemon zest and bring to a simmer. Let simmer for about 15 minutes, until the vegetables are very tender. Stir in the spinach and parsley and cook until it is wilted.
Either with an immersion blender in the pot or in a countertop blender (in batches), puree the soup. Return to the pot if necessary and then stir in the sour cream. Taste and adjust seasoning as needed. Serve hot.