In a large Dutch oven, combine the sausage and onion, breaking apart and browning the sausage until it’s cooked through and the onions are soft.
Once the sausage is done cooking, stir in the minced garlic and cook for two to three minutes or until it browns slightly and starts to smell nice.
Start sprinkling in the flour, mixing it into the sausage to that it soaks up any grease. Keep adding it, a little at a time, until the grease is completely incorporated into the flour. Let it cook, continuing to stir, for about two minutes to cook the flour taste out.
Stir in the chicken broth and let that cook for a couple minutes as well.
Cut the baked potatoes into bite-sized pieces (we included the skin in ours) and add them to the pot along with the mashed potatoes (Both can be cold at this point. No need to heat them up beforehand.). Also toss in the fennel, garlic salt and pepper.
Stir the mixture well. Start by mixing in about one cup of milk and let the soup cook, stirring regularly so it doesn’t scorch on the bottom, for five to 10 minutes or until it begins to thicken. If it’s too thick, add more milk until you reach your desired consistency.
Once the soup is heated all the way through (probably another 10 minutes or so), dump in the sharp cheddar and pepper jack cheeses and stir until it’s melted.
Serve hot with a bit more cheese on top if desired.