Heat lard in a large soup pot, add onion and garlic and cook until tender, about 3 minutes. Add ham and cook for 2 minutes. Add greens, a handful at a time, stirring them into fat until wilted before adding next bunch. Add stock, bay leaf, 1 teaspoon salt, cayenne and thyme . Bring to a boil, reduce heat and simmer 1 hour. Taste for seasoning -- greens should be tender and slightly spicy; add enough salt to enhance flavors. To thicken, have gumbo bubbling gently and stir in 1 tablespoon file powder to start; add file a little at a time until thickened; simmer 3 minutes more. Serve with steamed rice and green onions