Crush the fennel and cumin seeds using a pestle & mortar.
In a mixing bowl, combine the mince and the spices, season with salt and pepper. Form two or three balls, flatten these slightly between your palms. Put them on a plate and chill until needed.
Turn on your grill or BBQ. Brush the patties with some oil and cook them over medium heat for 3-5 minutes. Flip the burgers, cover them with mozzarella slices if you want cheese on top. Cook for 3-5 minutes. When the cheese is melted and the burgers are cooked through, transfer them to a plate.
Cut the buns in half, toast them, then spread mayonnaise on bottom half, layer lettuce, tomato, onion and pickled cucumber and add the burger. Cover with the other half of the bun and serve.