How can I recover deleted text in Notes app on my iPhone 11?

If you have not done anything else since or even exited the note, shake the phone and then tap on Undo in the pop up that appears or tap on the yellow backwards arrow button at the top of the Note screen to undo the last action.

Use Notes on your iPhone, iPad, and iPod touch - Apple Support

If you did something else, or wrote something else already then there's no way to get the text back.

And the back arrow?

for the shake to undo, make sure the option is turned on in Settings ➜ Accessibility ➜ Touch ➜ Shake to Undo is turned on.