In a large pot or Dutch oven, melt the butter over medium heat. Add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic and cook for another minute until fragrant. Stir in the sliced smoked sausage and cook until browned, about 5-7 minutes.
Sprinkle the flour over the sausage and onion mixture and stir to combine. Cook for 1 minute, stirring constantly.
Gradually whisk in the chicken broth until smooth. Add the cubed potatoes, salt, and pepper. Bring to a boil, then reduce heat and simmer, covered, for 15-20 minutes, or until potatoes are tender.
Stir in the milk and heavy cream. Heat gently until warmed through, but do not boil.
Taste and adjust seasoning if necessary. Serve hot.
Notes
Garnish with fresh parsley or chives if desired. This chowder is delicious served with crusty bread.