Unfortunately, you can search code by file extension and phrase, and you can use advanced search to search for repository descriptions filtering by stars, but I don't believe you can do both at once.

For instance, searching for "flask" and limiting the results to >1000 stars returns only the 27 repositories with a matching description[0], but the code search returns over 4 million results, ignoring the stars parameter[1].

https://github.com/search?l=&q=flask+stars%3A%3E1000&ref=adv...

https://github.com/search?l=&q=flask+stars%3A%3E1000&ref=adv...