Melt the butter in a medium pan over medium heat. Add the diced onion and cook until soft and translucent, about 4 minutes.

Stir in the peas and salt and cook until the peas are soft but still bright green, about 2 to 3 minutes. Add the lemon juice, stir and remove from heat. Stir in the Parmesan and freshly ground black pepper. Serve warm.