I tried a manual shutting down with Fuel Valve on FL350. It gave me an error message on EICAS, to descend to FL250. But when I turned on Continous Ignition and Both starters buttons, I could spin it up mid-flight even at high altitude. I did not changed any fuel config.
I do not know if it is a real simulation, but it just worked. Also it is important on the ground to shut down air bleed while engine is started.