At first site I thought the plant was a rumex. After examining the inflorescence and leaves, I came to the decision that it must be Chenipodium polyspernum, even though the stalk is not red flushed but only streaked. If I am wrong, I do hope a member will correct me. Best wishes to all members.
Could be Chenopodium bonus-henricus, but I am not sure. Chenopodium species are too variable and overlapping.
Re: Chenopodium Hello Andrey, I sent the photographs to my friend who is the Master Gardener of the Botanical Gardens of the Università di Camerino. He said it is Beta vulgaris subsp. vulgaris. The flowers are so similar to a Chenopodium! My thanks and best wishes, duffy
Beta is closely related to Chenopodium, too - so it's easy to see where you were getting the feeling!