Preheat oven to 425°F. In a large saucepot, melt butter over medium heat. Add leeks and cook until softened, about 6 minutes. Stir in greens, cover and cook 8 to 10 minutes or until just wilted, stirring occasionally. Cook greens in batches if your pot doesn't hold them all at once. Transfer greens mixture to a bowl and set aside. Add milk to saucepot and heat just to a simmer. Remove from heat, crumble in goat cheese and whisk until melted. Stir in greens mixture until combined, then transfer to a shallow 3-quart baking dish. Bake 10 minutes, then top with cracker crumbs and continue baking until crumbs are toasted and greens are heated through, about 10 minutes longer.