The verb is the past progressive verb, "was cooking."
Cooking, like a lot of -ing words, can serve as a noun. When it does, it's called a gerund.
Cooking is difficult.
Cooking brings me joy.
I'm no good at cooking.
(It can be an adjective, too, but let's not borrow trouble.)
But "is cooking" and "was cooking" (along with will be, shall have been, yada-yada-yada) is using the word cooking as a verb.