今日のランチは

as for today's lunch, …

→ 今日
phrase Rare #42,943
as for today's lunch, … Introduces today's lunch as the topic; the rest of the sentence says something about it.
JA: 今日のランチはカレーにしよう。
EN: Let's have curry for lunch today.

今日のランチはピザにしない?

How about pizza for lunch today?

今日のランチは俺が出すよ。

Lunch is on me today.

今日のランチは、私が払うわ。

Lunch is on me today.

今日のランチはピザにしましょう。

Let's have pizza for lunch today.