I first cooked vegetable waffles when I worked in Paris. They were made of potato blended with shredded vegetables and were used to garnish game dishes. Since those days I have experimented with all manner of vegetables. Corn and squash have been successful, as has this combination of celeriac and smoked salmon.
Peel the celeriac and cut it into chunks, then cook in the boiled milk for 20-25 minutes until tender. Drain well, reserving the milk. Pass the celeriac through a sieve or mash until smooth. Put it in a bowl with the eggs and egg yolk, flour,
Heat a waffle maker until almost smoking and then add a quarter of the butter to each waffle compartment. Pour in a small ladleful of the mixture, close the lid and cook as for normal waffles. Place a good spoonful of horseradish cream on each warm waffle and top with the smoked salmon and caviar. Serve immediately.