Celeriac and Portobello Soup with Cumin

Celeriac and Portobello Soup with Cumin
(serves 6)
2 tbsps butter
1 tbsp ground cumin
1/2 tbsp cumin seeds
several grinds fresh black pepper
1 head of celeriac, peeled and diced
4 Portobello mushrooms, diced
1 white onion, diced
3 cloves garlic, crushed
1 litre chicken or vegetable stock
1/2 cup (125ml) cream for drizzling
handful chives, finely chopped, for garnish