In a large pot over high heat, add wine and mussels then cover pot with lid and steam until mussels just start to open (discard any that don’t). Remove from heat and leave mussels to cool.
When cool enough to handle, debeard mussels (see below) then separate mussel meat from shells. Remove tongues and main (round) muscle then cut mussels in halves or thirds, depending on size. Set aside.
In a small bowl, combine mascarpone, lemon zest and juice and set aside.
In a separate bowl, add flour, salt, baking powder and pepper and mix to combine. Stir in eggs and milk to create a thick batter. Add parmesan, onion and mussel meat and mix through batter.
In a non-stick pan over medium-high, heat a splash of the olive oil until hot. Add a generous spoonful of batter for each fritter and cook until golden brown on both sides and firm to the touch.
Repeat in batches, adding more olive oil as you go, until all batter is cooked. Keep fritters warm or reheat in microwave before serving.
In a non-stick pan over medium-high, heat canola oil until shimmering. Add capers and fry until crispy. Remove and drain on kitchen paper towels.
To serve, place fritters on a board draped with baking paper. Top fritters with lemon mascarpone and sprinkle with fried capers. Garnish with pea tendrils (if using).