Add chopped onion and cook 2–3 minutes until fragrant and softened.
Cook Vegetables & Thicken
Stir in chopped celery and salt; cook 8–10 minutes until pale green and slightly caramelized.
Add chopped potato and minced garlic, tossing to coat.
Sprinkle flour over the vegetables and stir briskly for 1 minute.
Simmer & Finish
Pour in chicken broth and bring to a gentle simmer; cook 10–15 minutes until potatoes are tender.
Blend soup in batches until smooth, return to pot, and stir in whole milk, heavy cream, and pepper; warm through without boiling.
Notes
You can substitute olive oil for butter for a lighter version.If you prefer a chunkier texture, reserve some sautéed celery and stir in after blending.