Start by prepping everything before cooking.
Add a little oil to a pot on medium high heat and sear the sausage. Remove the sausage once seared then add in the onions and garlic. Add a little more oil if needed then cook until softened and light browning. Note, you can add light salt and pepper throughout the cook or wait until the end.
Lower the heat the medium, then add in the butter. Once the butter has melted, toss in the flour then begin cook the flour with the onions and garlic. After a minute, slowly begin to pour in the chicken broth and milk. After adding both, add the potatoes and sausage into the soup.
Cover the pot and turn on medium-low heat. After every 5 minutes or so, stir the pot to prevent sticking. It'll take about 15-20 minutes for the potatoes to soften, depending on how small you diced them. When they are soft, turn off the heat. Pour in the heavy cream and lastly taste for salt and pepper.
When finished, you can blend a portion of the soup in a blender or use an immersion blender. Only a little bit if you like a chunky soup but blend everything if you like the soup without chunks and creamy.
Allow to cool a bit before serving and enjoy.