Melt the butter in your Dutch oven over medium-high heat. Add the chopped onion and cook for about 3 to 5 minutes, stirring occasionally, until it turns soft and slightly translucent. This step builds the flavor foundation for the entire pot, so don't rush past it.
Lower the heat to medium-low and stir in the flour, letting it cook for a minute until lightly golden. Slowly whisk in the chicken stock, then add the potatoes, sweet potatoes, corn, and nutmeg. Bring everything to a boil, then reduce to low and simmer uncovered for 15 to 20 minutes, until the potatoes turn fork-tender.
Stir in the shredded turkey and half-and-half, letting the soup simmer gently for another 5 to 6 minutes until it's heated all the way through. Take the pot off the heat once everything is warm, since you don't want it boiling in the next step.
Gradually stir in the shredded cheddar, a handful at a time, until it fully melts into the soup. Adding it slowly off the heat keeps the texture smooth instead of grainy. Season with salt and pepper to taste before serving.
Ladle the hot soup into bowls and top with salted pepitas and a dash of hot sauce if you like a little kick. Serve immediately while it's warm, creamy, and at its cheesiest best.