Heat a non-stick frying pan on high. Melt half of butter and cook potatoes, stirring for 5 minutes until golden. Remove from pan and melt remaining butter. Add half of egg. Using a silicone spatula, pull egg mixture from outside edge towards centre to let uncooked mixture take its place to cook. Top half of omelette with half of salmon, potatoes, mascarpone and quarter of spinach. Season to taste and flip over to enclose filling. Slide onto a warmed plate.
Repeat with remaining ingredients to make second omelette and serve garnished with dill, along with remaining baby spinach and lemon wedges.