Slow cooker lentils with chicken sausage, spinach and feta cheese

brown lentils

homemade or low-sodium chicken broth

leek, white part only, trimmed and chopped

pre-cooked chicken sausage (I use fontina and spinach flavor or roasted garlic flavor), sliced into rounds

fresh black pepper, or more to taste

Kosher salt,

fresh baby spinach leaves

crumbled feta cheese, or more to taste