Briefly sauté the onion in the butter, add the beetroot, sauté for 1-2 minutes and deglaze with the broth. Cover and simmer gently for 10 minutes, then puree.
Add the juice of the squeezed orange, about 1 third of the zest, salt, pepper, cream and the Grand Marnier to the soup. Heat again for 5 minutes, season with orange zest / Grand Marnier if necessary.
Arrange in plates and garnish with (homemade) croutons and the remaining orange zest.