In a heavy stock pot, heat oil and sauté onions until lightly golden, about 8 minutes. Add squash and ginger and continue braising for another 5 minutes. Add broth, cover and simmer for about 20 minutes or until squash is tender. Transfer portions of this mixture to a blender and puree. After entirely pureed and returned to the same pot, add the tomato puree and cayenne. Season to taste with salt and pepper. Continue simmering to blend flavors, adding more broth if soup is too thick. Just before serving, toast cumin and mustard seeds in remaining teaspoon of oil or about 2 minutes or until fragrant. Ladle soup into bowls, garnish with toasted seeds and serve piping hot.