1)    If one sees lightning and hears thunder simultaneously, one says “Oseh Ma’aseh Bereishis” on both together. If one “Shecocho ugevuraso malei olom” on both. one has also fulfilled one’s obligation. However, one should not make both blessings together. (Shulchan Aruch and Mishne Berurah 227)

2)    However, if they were not simultaneous, he should first make the blessing “Oseh Ma’aseh Bereishis” on the lightning, and then “Shecocho ugevuraso malei olom” on the thunder. (The explanation of the latter blessing is “Blessed is He who gives nature the force to show the power of the Creator of the Beginning so that they [all creatures] should fear Him!) (ibid)