Preheat oven to 400.
Add butter into a large skillet on the stove and let melt. Add collard greens, onion, and garlic once butter is melted.
Add 1.5 cups of chicken stock to collard greens (reserve the rest for another part of the recipe). Cover and cook on a simmer for 15 minutes.
Cut potatoes into your thin slices while you wait.
Butter a medium sized baking dish (not quite as big as a 9x13).
In a large bowl combine collard greens (with liquid from pan), potatoes, salt, pepper, and 1 cup shredded cheese. Place mixture into your baking dish. Pour in milk and remaining chicken stock. Add remaining cheese on top of mixture.
Cover and bake for 30 minutes, or until dish is bubbly.