De-shell and blanch sweet peas in a saucepan with salted, boiling water, cooling them in a bowl of ice water when just tender.
In a blender or bowl with an immersion blender, combine the cooled peas, Mascarpone, green onion, lemon juice, salt, pepper and olive oil. Blend until smooth.
The mixture should be thick and have a texture resembling hummus.
Slice and toast country bread until golden brown and drizzle with olive oil and a pinch of fleur de sel.
Using a soup spoon, serve 3 large spoonfuls onto each slice of bread and, keeping volume, distribute the pea purée onto each slice.
Top the pea purée with 2 slices of jambon/prosciutto and garnish with fresh mint and fleur de sel.
Serve immediately.