Cut the bread into 5 cm (2 in) dice and put them in a bowl. Pour the warm water over the bread and set aside to soak for 5 minutes until soft.
Gently squeeze the excess water out of the bread. Put the bread in a blender with the cod roe, lemon juice and shallots. Finely grate in the garlic using a microplane. Blend on high speed until combined, then, with the m