In a large stock pot heat cumin, caraway and fennel seeds over medium heat until lightly toasted and fragrant; do not burn. Remove to a small plate. Add oil and heat over medium-low heat. Add leeks and cook until soft, about 5 minutes. Add potatoes, ham and stock, bring to a boil, then add cabbage and reserved seeds. Reduce heat and simmer until potatoes are tender, about 15 minutes. Season to taste with salt and pepper.