Brown sausage with diced onion in large pot over medium-high heat for 8-10 minutes until sausage is cooked through and no longer pink. Drain excess grease, leaving 1-2 tablespoons.
Add minced garlic and cook for 30 seconds to 1 minute, stirring constantly, until fragrant.
Slowly add chicken broth, using wooden spoon to scrape up browned bits from bottom of pot for extra flavor.
Add diced potatoes, salt, and pepper. Stir to combine.
Bring to boil, then reduce to simmer. Cover and cook for 20-25 minutes until potatoes are fork-tender.
In small bowl, whisk together heavy whipping cream and cornstarch until smooth with no lumps.
Increase heat to medium-high. Stir cream mixture into hot soup continuously. Soup will thicken within 3-5 minutes.
Let soup heat for 3-5 minutes until cream is fully incorporated and soup has thickened.
Taste and adjust seasoning. Serve hot topped with parmesan cheese.
Notes
Stovetop Method: Total cooking time about 40-45 minutes including browning sausage.
Slow Cooker Method: Brown sausage first, then add all ingredients except cream to slow cooker. Cook LOW 6 hours or HIGH 4 hours. Add cream last 15 minutes.
Drain excess sausage grease but leave some for flavor.
Scrape browned bits from pot bottom for extra flavor.
Russet potatoes create creamiest texture and help thicken soup naturally.
Make cornstarch slurry smooth before adding to prevent lumps.