Lamb steak sandwich with garlic, lemon and parsely butter and Brie

2 boneless lamb steaks

2 tbsp wild garlic, lemon and parsley butter (see belowO

1 tbsp mayonnaise

4 medium chestnut mushrooms, sliced thinly 

4 thick slices of good young Brie (Wicklow Bán Brie)

large sourdough baguette

For the butter:

125g butter

25g wild garlic leaves, chopped finely (or one clove of garlic, crushed)

pinch of sea salt

10g fresh parsley, chopped

juice of ½ lemon