Place slow cooker liner into slow cooker, if using.
place potatoes, vegetables, garlic, rosemary, sage, salt and pepper into slow cooker
add onion broth, cover and cook on low for 5 hours or so
when potatoes and veggies are soft (after about 5 hours or so), carefully use an immersion blender to combine all ingredients. The longer you use it, the smoother the final soup will be.
30-45 minutes before serving, mix about 1 cup of half and half with a few tablespoons of hot soup in a separate bowl to temper the half and half. set aside.
add shredded cheddar into slow cook and stir as it melts to fully combine
Add sour cream and tempered half and half mixture into slow cooker, stir to combine and cover.
keep on low for another hour or so and then set to warm until ready to serve.