Kirill Belorusov, 32, fled to his native Estonia after killing Laureline Garcia-Bertaux, 34, on March 3 or 4 this year, jurors heard.
Officers found her naked body wrapped in bin bags in Kew on March 6 after friends raised concerns over messages he sent from her mobile phone and examined a flower bed in the flat, the Old Bailey was told.
Oliver Glasgow QC, prosecuting, said: “After carefully removing the earth they discovered that a body had been buried in the garden.
“It was Laureline Garcia-Bertaux. She was naked; her feet were bound and her hands were tied behind her back; there was a ligature around her neck; and she had been wrapped in bin bags.
“Once her body was examined it became apparent that she had been strangled to death and buried in the garden at some point during the weekend that Kirill Belorusov had spent with her.”
Belorusov, who denies murder, had broken up with her months before and got in touch claiming he had found a new home for her and wanted to pay back money he owed her, the jury heard.
His DNA was found on a ligature tied around her neck after he was arrested in Estonia six days later and extradited back to the UK on March 20, the jury heard.
Mr Glasgow said he had accessed the internet via his phone at his alleged victim’s home on March 4.
He added: ” In other words, having killed her and buried her in the garden, he sent false messages from her phone to cover his trans and remained at her home address until he thought it safe to leave on the Monday morning.
“Everything he had said to her friends and the police was a lie, and he lied because he had murdered Laureline Garcia-Bertaux.
“It is the Crown’s case that, no matter what may have happened during the weekend that Kirill Belorusov spent with Laureline Garcia-Bertaux (double corr), there is nothing that this defendant can say to excuse what he did to her.
“Kirill Belorusov tricked Laureline Garcia-Bertaux into believing that he was going to help her, he deceived her with promises of settling his debt to her, and he lied to her about the house he had found that she could move into.
“It was only inevitable that Laureline Garcia-Bertaux would get suspicious about what was going on; after all, she had only been given very limited details about the property and no removal struck ever turned up.
“Quite how Kirill Belorusov thought he was going to play out this charade remains to be seen but clearly at some point he decided to kill Laureline Garcia-Bertaux.
“Having killed her he did all he could to get away with murder: he buried her body in the garden in order that he could leave the country before she was found; he sent messages to her friends in order that they might believe she was alive and well.
“He expressed concern to those same friends when they contacted him to ask if he knew where she was; he sent messages to her phone begging her to get in contact with him; and he told everyone who got in touch with him that he would do all he could to help find her.
“It is difficult to conceive of someone behaving in a more calculated or callous manner.
“The defendant denies playing any part in her death and insists when he last saw her she was alive and well.”
Belorosov appeared in the dock in a grey prison t-shirt, clean-shaven with a completely shaven head.
Ms Garcia-Bertaux was originally from Aix-en-Provence in France.
As well as working as a personal assistant in London, she was involved in film and had worked with Dame Joan Collins on a 2018 short called Gerry.
Her mother and father and other family members travelled from France to attend court.
The trial is ongoing.