Liberally spray the bottom and sides of a 6-quart slow cooker with cooking spray. Cube the ham, dice the onion and mix together the 2 types of cheese.
To prepare the sauce, pour the chicken broth, half and half, softened cream cheese, grated Parmesan cheese, melted butter, garlic salt, black pepper and ground mustard into a stand blender. Process until smooth and fully combined.
Separate all ingredients into thirds. Begin layering 1/3 of the sliced potatoes in the bottom of the slow cooker. Sprinkle with 1/3 diced onion and 1/3 of the cubed ham. Drizzle with 1/3 of the sauce and 1/3 of the shredded cheese. Repeat ending with sauce and cheese.
Cover the opening of the slow cooker with doubled paper towels then place the lid firmly on top.
Cook on high for 3-4 hours or on low for 6-8. Remove the crock from the slow cooker, sprinkle with chopped chives and rest for 10 minutes before serving.