Where does the Bible show us that a woman was a pastor?
Where does it call anyone a pastor. But we do see that women were teachers.
We can look at Junia in Rom 16:7.
Regarding Junia
An attempt should be made to answer the following questions with respect to which opinions vary:
a. Should we read Junias (masc.) or Junia (fem.)? In the latter case Andronicus and Junia could be husband and wife.
b. Did Paul say, “my fellow-countrymen” or “my relatives”?
c. Does “who are outstanding among the apostles” mean “outstanding in the estimation of The Twelve” or does it mean, “who, as apostles, are outstanding”?
Baker's New Testament Commentary
Rom_16:7, he designates Andronicus and Junia, his kinsmen and fellow-prisoners who were distinguished among the apostles and who had been converted before Paul, that is, before 34 a.d., at least eight years before it was ever pretended that Peter was in Rome. Other persons are also mentioned as distinguished, and it is not improbable that they were the early founders of the church at Rome (Rom_16:12-13, etc.)
Barnes
Of note among the apostles.—An ambiguous expression, which might mean, and, judging by the word alone, would perhaps more naturally be taken to mean, “distinguished as Apostles themselves.” This sense is not to be disregarded as absolutely impossible, for the title “Apostles” does not appear to have been limited to the Twelve.
Ellicott
If it is feminine and if the phrase "the apostles" referred to a wider use of that term than "the Twelve," then this was a lady apostle.
It is also interesting that the spelling "Junias" has not been found anywhere in Roman literature or inscriptions, but the name "Junia" was very common.
Utley
According to the broader application of the term, such men as Barnabas, Epaphroditus, Apollos, Silvanus, and Timothy are all called “apostles.” They all evangelize. They can be described as missionaries or itinerant Christian evangelists.
So the question is did these men as apostles teach other men and if they did then they would logically have had authority over them.
Why would it be any different for Junia and other women that we are told taught?