Learn German: Das Mittagessen ist heute kalt.

Mittagessen is a compound noun made of der Mittag (noon) and das Essen (food or meal). In German compound nouns, the gender is always determined by the very last word. Since it is das Essen, the entire word takes the neuter article das.

Why doesn't the adjective kalt have an ending here, like kaltes?

In German, adjectives only take an ending when they are placed directly in front of a noun, such as ein kaltes Mittagessen. Because kalt comes after the verb ist to describe the noun from a distance, it stays in its basic dictionary form without any ending.

Could I put heute at the very end of the sentence like we do in English?

It is much more natural in German to keep heute right after the verb. Unlike English, German word order strongly prefers to place time words early in the sentence, typically right after the main conjugated verb.

Can I start the sentence with heute to emphasize that it's only cold today?

Yes, you can! However, remember that the verb must always stay in the second position. If you move heute to the very front, the subject gets pushed to the other side of the verb. The sentence would become: Heute ist das Mittagessen kalt.