The newly discovered painting is by Hendrik Willem Mesdag (23 February 1831 – 10 July 1915). He made it in 1854 for his fiancee (later: his wife), Sina van Houten, also a painter.
On the back of the watercolour, Hendrik Willem wrote a poem for Sina.
The words of the poem are (translated):
Thoughts are not subject to laws;
therefore; think of the
maker of this; as often as ye will take up this sheet.
Gron[ingen], July 1854
H W Mesdag
Hendrik Willem Mesdag and Sina van Houten are most famous for painting the very big Panorama Mesdag. So, this very small work is a bit of a surprise.