Remove the biggest ribs from the beet, wash and roughly slice up all the greenstuff on a board, put it in a large bowl with water. Chop up the aromatics and simmer them in butter till they begin to colour. Put in the undrained greenstuff, with the tomatoes and potato, add salt and pepper and boil up, stirring. When the greens have reduced in volume, cover with hot water and cook briefly till tender, then pass through a sieve.
Stir some fresh butter into the purée and, if you like,