Preheat the oven to 200 °C / 400°F.
In a bowl, combine the softened salted butter, finely chopped parsley, crushed garlic, 50 grams Parmesan (reserve a bit extra to sprinkle over the loaf), a squeeze of lemon juice if using, and a pinch of salt if desired; mix thoroughly to combine.
Slice the loaf of ciabatta or large baguette into fairly thick cuts without cutting all the way through so the slices remain attached. Spread the butter mixture generously between each cut, sprinkle the extra Parmesan over the top, and wrap the entire loaf tightly in foil.
Bake the wrapped loaf for 20 minutes, then open the foil and bake for a further 5 minutes to brown the top.