Chenopodium album
Some seeds usually made into gruel in historical times are acorn and manna grass ("Glyceria fluitans"). Roots of the sea beet, Beta maritima, were prepared as well. That species is ancestral to modern domestic beets. Greens could have been boiled from nettle ("Urtica dioica"), orache ("Atriplex") and goosefoot ("Chenopodium album").