The time-frequency methods available in Brainstorm are described in this tutorial:
https://neuroimage.usc.edu/brainstorm/Tutorials/TimeFrequency
If you install FieldTrip (or use the compiled version of Brainstorm), you would also be able to use the FieldTrip multitaper approach (function ft_mtmconvol):
http://www.fieldtriptoolbox.org/tutorial/timefrequencyanalysis
I'm not sure how this relates to the TSE method available in BESA, you might need to read the methods articles published about this method.
@Sylvain @hossein27en @leahy Any suggestion?