Preheat the oven to 200C/180C Fan/Gas 6.
Grease 6 holes of a large muffin tin with a little oil, then line with long strips of baking paper. Leave the ends overhanging on either side, to help lift out the röstis.
Shred all the root veg, using the fine grater on a processor, or a spiraliser. Toss in a large bowl with the oil, ½ the chives and some pepper. Mix with 1 egg, to bind.
Divide the mixture between the lined holes of the tin, pressing to fill any gaps. Bake for 25-30 mins, until crispy and golden brown at the edges, then remove from the oven. Push down the centres with the back of a spoon to form a shallow ‘well’ in each rösti. Allow to cool in the tins for 10 mins.
Line a baking tray with baking paper. Arrange the bacon rashers on it, spaced apart, then bake on the shelf below the röstis for 12-15 mins, until crisp. Cool on the tray for5mins, then transfer to a plate lined with kitchen roll.
For the poached eggs, bring a small pan of water to a simmer. Crack 1 egg into a mug, taking care not to break the yolk. Swirl the water and pour the egg into the centre of the pan; turn the heat down to a gentle simmer.
If making ahead, cook for 2 mins, or until just set and firm to the touch. If making to serve immediately, cook for 3 mins. Remove with a slotted spoon and put on a plate lined with kitchen roll. Repeat with the remaining eggs, cooking one at a time.
Lift the rösti nests out of the tin, peel off the paper and put on a baking tray. Add a poached egg to the centre of each one.
If making ahead, chill in the fridge for up to 1 day and reheat in the oven, along with the bacon, at 170C/150C Fan/Gas 3, for 7-10 mins, until warmed through.
Just before serving, make the hollandaise according to the pack instructions, using the milk. Stir in the Dijon mustard if using.
Put one nest on each plate, top with the hollandaise and bacon, and season with pepper. Serve with the watercress on the side.