What drives N1 during a jet engine start? A typical jet engine start sequence that I understand is as follows:

ENG Start Switch to Ground → Start Valve open → N2 increases → N1 increases → ENG Start lever to Idle (when N2 has reached a certain number) → Fuel flow starts → EGT rises → ENG Start Switch cutout 

As far as I understand, N1 and N2 are mechanically seprated. If that's the case, what drives N1 before it starts to rotate from the following combustion?
N1 is driven by the starter flow through windmilling. N2 is started through a gearbox as described in this answer, driven by an air starter or by an electric motor. This causes air to start flowing through the engine, and the LP turbine blades pick up energy from this airflow.



Above graph is from The Jet Engine by Rolls Royce, and shows that the LP spool (N1) starts to turn in the dry cranking (starting) phase, after N2 has been accelerating for a while. The speed with which N1 spools up depends on the engine type, but even the large inertia of a helicopter rotor is slowly starting to be driven around by the starter flow.