"Our Beloved Sudan" weaves the personal story of one family with the history of the country shown through archival footage and interviews with key political figures in the lead-up to partition last year.
Predominantly black African and Christian South Sudan became independent from the Arab-dominated Muslim north in July 2011 after a nearly unanimous vote in a referendum.
The vote came under a 2005 peace deal which ended 23 years of civil war that killed about two million people and displaced millions more.
Elsanhouri said her motivation as a film-maker came from growing up in Britain where, as a university student wanting to learn about her ancestral home, she found diaries of colonial administrators but no Sudanese voices.
"I felt very frustrated by this, and I thought... did they not leave any footsteps in history?" she told AFP in an interview.
"I guess that upset me a lot," she said. "So I think my whole project, my life project, is about creating these footsteps."
Her latest production is not just a film but an act of historical documentation, said Elsanhouri, who moved to Britain when she was nine.
"I want, 50 years from now, if a researcher is looking for something about the partition, to find something by a Sudanese," she said before the film's Khartoum screening.
The 90-minute production features Amira, a young woman with a northern father and a southern mother.
It chronicles the impact of the civil war and the lead-up to independence on Amira and her ethnically-mixed family, alongside interviews with prominent political figures from the north and south.
-- 'You cannot be silent' --
Journalists and press freedom advocates say a media crackdown intensified in Sudan after South Sudan separated but Elsanhouri said she did not face any official obstacles "because I work in a very quiet and unconfrontational way."
The British citizen also admitted to having "less to lose" than a Sudanese living permanently in the country.
The real obstacle in making the film came in trying to establish a bond with her southern characters.
"They trusted me up to a point but ultimately I am a northerner, and they could not give me their testimony," she said. "I really came face to face with the extent of the divide between us as a people."
"Our Beloved Sudan" has not yet been shown in the South.
The film cost about 50,000 euros ($64,000), much of it from her own pocket and some from the European Union.
"We have funded this movie as (an) expression of our will to bring better understanding" between north and south, EU ambassador Tomas Ulicny told more than 200 people who filled an outdoor terrace at Khartoum's Goethe-Institut for the screening.
Amira, the film's main character, was among the audience for the documentary which had its global premiere late last year at the Dubai International Film Festival.
Elsanhouri said that although she had wide access to politicians -- including former prime minister Sadiq al-Mahdi and the wife of the late southern leader John Garang -- a key voice was missing.
Vice President Ali Osman Taha, who analysts say is a possible successor to President Omar al-Bashir, would not give Elsanhouri an interview.
"And that saddens me because I think you cannot be silent," she said.
"Fifty years from now when we wanted to know how did this country come to part, what were the processes, what were the internal dynamics, what was it that Mr Osman thought? This will be lost to history."
More than a year later, the anticipation and sadness which preceded Sudan's division has not been replaced with needed reflection, she said, describing the partition as an inevitable outcome of history.
"So really the film is in a way saying, how can we learn from the past?" she said.
"I'm not looking at the geopolitics. I'm saying: As Sudanese, what can we be responsible for?... How can we look at ourselves and take responsibility?"
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