Question: I learned as a child that one should only recite the beracha upon seeing lightning if they have also heard thunder. Is there any basis to this?
Answer: The Gemara (Berachos 59a) teaches that there are two berachos to be recited upon seeing lightning and hearing thunder though there is a machlokes as to which beracha should be recited for each.
Tosafos (Berachos 59a) write that one recites either she’kocho ugvuraso malei olam or oseh ma’ase bereishis while Raavad (Berachos 10:14) writes that one should recite both berachos.
The Taz (OC 227:1) and Mishna Berura (227:5) write that the minhag is to recite she’kocho ugvuraso malei olam when hearing thunder and oseh ma’ase bereishis when seeing lightning (which better demonstrates Hashem’s might). Some people do not recite a beracha after lightning until they hear thunder, too, though this practice is wrong, as the beracha must be recited toch kedei dibbur, within a couple of seconds (ibid. 227:12). If one sees and hears them together, one only says one beracha, preferably oseh ma’ase bereishis.
While Ashkenazim say the full beracha with Hashem’s name, some Sefardim follow the Kaf Hachaim (OC 227:1) who writes that one should recite the beracha without Hashem’s name. Other Sefardim follow R’ Ovadia Yosef (Yechave Daas 2:27) and recite the full beracha.
In conclusion, if one sees lightning and hears thunder close together, one recites the beracha of oseh ma’ase bereishis. Otherwise, one recites a separate beracha on each.