Place sausage and onion into a large stockpot over medium heat. Break sausage up with a spoon and cook until onion is translucent and sausage is browned and crumbly, 7 to 8 minutes. Drain if desired.

Add chicken broth and potatoes; bring to a boil. Reduce the heat to medium-low and simmer until potatoes are tender, 10 to 15 minutes. Stir in spinach and cook until wilted, 1 to 2 minutes.

Remove from the heat and stir in evaporated milk. Taste and season with salt and pepper as needed.