Dice onion and slice carrots, celery, and potatoes.
Melt 1 tablespoon butter in a large soup pot over medium heat. Brown sausage, breaking it into crumbles. Transfer to a paper towel–lined plate.
Melt remaining butter in the same pot. Add onions, carrots, and celery with a pinch of salt and pepper. Cook 4 minutes, stirring often.
Stir in garlic and thyme; cook 1 minute.
Add broth and potatoes. Bring to a medium-high simmer and cook ~15 minutes, stirring occasionally, until potatoes are tender. Add broth as needed.
Reduce heat to medium-low. Pour in heavy cream. Scoop out about 2 cups of soup (including potatoes & veggies) and blend until smooth. Return purée to the pot and stir.
Add cooked sausage back in, along with nutmeg and salt & pepper to taste. Simmer a few minutes more before serving.
Notes
Remove sausage from the pot after cooking to prevent overcooking and rubbery texture.
Blend a portion of the soup and potatoes to thicken the soup slightly and give it a chowder-like texture.
If you don't have fresh thyme, dried works as well. Use about ½ tsp.