In a 4-quart saucepan, heat butter over medium heat. Add onions and garlic and sweat for 3 minutes until vegetables are translucent and aromatic. Add chicken stock and cream and bring to a simmer. Whisk in polenta and reduce heat to low. Cook over a low heat for 2 hours, stirring regularly.
Remove from heat and whisk in cheese and butter, and season with salt and serve.