Fry the bacon in a dry frying pan until crisp. Drain on kitchen paper and roughly chop.
Put the potato and parsnip on a clean tea towel and squeeze over a bowl to remove the excess liquid. Place in a separate bowl and mix in the bacon and parsley.
Melt the butter and oil in a frying pan over a medium heat. Add the potato mixture and press it down with the back of a spoon. Season with salt and pepper. Fry for 2-3 minutes on each side, using a fish slice to carefully turn it over, until crisp and golden-brown.
For the hollandaise, melt the butter in a saucepan over a low heat, then skim off the froth to leave a clear yellow liquid. Put the egg yolk, lemon juice and one tablespoon water in a blender. Turn on the machine and slowly pour the warm clarified butter through the lid. Add the mustard and salt and blend.
For the gammon, heat a large griddle pan over a medium-high heat and cook the gammon for 2-3 minutes on each side, until cooked through.
For the poached egg, heat a large saucepan of simmering water, add the salt and vinegar and spin the water. Crack an egg into the pool of water and cook for 2-3 minutes, or until your liking. Repeat with the second egg.
Place a rösti on a plate. Top with chargrilled gammon and a poached egg and pour over the hollandaise.