Swede and potato cakes with black pepper and cream sauce

500g swede

300g potatoes

25g butter

1tsp Maldon salt

2 tsp agar flakes

1 egg yolk

1 tbsp parmesan, freshly grated  

1 tsp spring onion, chopped 

flour, for coating

olive oil, for frying

For the sauce:

250ml double cream

2 tsp black peppercorns, cracked or coarsely crushed

salt, to taste

30g butter

2 tsp smooth Dijon mustard 

watercress sprigs, to garnish