Bootstrap a phase-tracking PV inverter without mains: possible? How?
While what DL says in comments about lineman safety issues is technically correct, a formal study of real world conditions indicates the lineman is more likely to die from tripping over your cat than from electrocution. (They did not use a cat-metric in the study but that was the order of result.)

What DL says about legality is correct  - a grid tie inverter is invariably required to have "anti-islanding" features and while creating an island that is isolated is potentially acceptable, creating an island that intentionally include an unknown number of neighbours and linemen is definitely not. Some administrations frown more heavily on people doing such things than others. I'd expect your administration to frown relatively heavily -but also that many people would do this.

IF you can achieve isolation then a motor generator with decent waveform may be enough. Means of anti-islanding and demands on the realness of the grid synced to vary quite widely.

It will be possible to delve within the internals of your inverter to make it islandable. You could die by two main means.

You can buy inverters that are made to provide islands at a modest percentage of full power  - usually good quality and duly costly. SMA Sunny island is one such and I think they may use SMA Sunnyboy inverters as island slaves. Looking at how SMA do do what they do may be educational.

Pseudo sine wave inverters of reasonable capacity are much lower cost than "pure sinewave" and less again that grid tie units. You may be able to LC or just L filter the output of these to a level acceptable to many loads. When using a cheap motor-alternator set long agoI found that a 1:1 isolating transformer tidied things up nicely.

You MAY be able to use an AC motor as an electro-mechanical grid stiffener to 'fool' an anti-islanding system,  - run up motor on whatever and then drop onto grid tie inverter output.

In the nearish future I wish to run anti-islanding inverters in true islanded modes and will be investigating means of doing this. I do not wish to power the neighbourhood or excite linemen.