In a large pot or Dutch oven, melt butter over medium heat. Add the chopped white onion, diced carrots, thinly sliced celery, and salt. Cook, stirring occasionally, until the vegetables are tender, about 6 to 8 minutes.
Sprinkle flour over the softened vegetables and stir to coat. Cook for 2 minutes, stirring constantly, to cook out the raw flour taste.
Slowly pour in the chicken stock, stirring as you go to prevent lumps. Add the chopped rotisserie chicken breast, wild rice blend, and poultry seasoning. Stir to combine and bring to a gentle boil.
Reduce the heat to low, cover, and simmer for 30 minutes, stirring occasionally.
Stir in heavy cream and continue to cook, uncovered, for another 15 minutes or until the wild rice is cooked through and tender. Taste and adjust seasoning before serving.
Recipe Tips
Use unsalted butter if you want precise control over the soup's salt level. Salted butter works, but reduce the added salt accordingly and taste carefully before the final seasoning step.
Avoid bouillon cubes or powder as a substitute for chicken stock. A good-quality boxed or homemade stock makes a noticeable difference in the depth of the broth.
If your grocery store carries a true wild rice blend (a mix of wild rice and long-grain rice), it will work well here. A blend that is entirely wild rice will require a longer simmer time.