How did Rashi know it was lentils?

Cute little timely vignette about R. Chaim Brisker in R. Aharon Rakefett-Rothkoff's 'The Rav: The World of Rabbi Joseph B. Soloveitchik' Vol. I pp. 231-233:

There was a melamed in Brisk who considered himself an expert on Humash. He developed a very funny and strange idea. He wanted to teach my grandfather Reb Chaim how to really understand Humash....He recognized that Reb Chaim was a gaon olam in Talmud, but the melamed felt that he knew Humash better than Reb Chaim. He constantly urged Reb Chaim to study Humash with him.

Reb Chaim was very tolerant and very careful about the dignity of man. He always tried not to insult of abuse anyone....and therefore the melamed continued to bother him.

Finally, Reb Chaim decided to disprove the melamed's position. It was Parshat Toldot [Gen. 25:19-28:9]....Reb Chaim told the melamed: "You are right. I do not know Humash. I studied the commentary of Rashi today and did not understand it. Can you explain it to me?"

The melamed got all excited and exclaimed: "Of course I will explain it to you. What is the difficulty?"

Reb Chaim quoted the sentence "Jacob simmered a stew, and Esau came in from the field and he was exhausted. Esau said to Jacob: 'Pour into me, now, some of that very red stuff, for I am exhausted'" [Gen. 25:29-30]. Rashi says that the soup was made of red lentils....

Reb Chaim asked the melamed how Rashi knew the soup consisted of lentils?...Where did Rashi find out that Jacob was cooking lentils?

The melamed responded that when you utilize the word "stew" [Gen. 25:29] it generally means lentils. Then the melamed pointed out that Esau requested "some of that very red stuff". "Very red stuff" must mean lentils.

Reb Chaim told the melamed: "I have a better proof that Rashi is right. Simpy go a few sentences further in the Humash. It states explicitly: 'And Jacob gave Esau bread and lentil stew'" [Gen. 25:34].

From then on the melamed stopped bothering Reb Chaim about Humash. He realized there was very little he could teach Reb Chaim about Humash.