Is there a function in Racket that returns a procedure's lambda-expression?

No. Racket's lambda produces a closure that does not remember its S-expression (or syntax object) form. It does typically remember its name (or its abbreviated source location, if no name can be inferred), and that's often enough to help with debugging. (See object-name.)

You can build your own variant of lambda that has this feature, using Racket's applicable structures and a simple macro. Here's a basic example:

#lang racket

(struct exp-closure (f exp)
  #:property prop:procedure (struct-field-index f))

(define-syntax-rule (exp-lambda formals . body)
  (exp-closure (lambda formals . body)
               (quote (exp-lambda formals . body))))

(let ([my-thunk (exp-lambda () (+ 1 2))])
  (printf "fun is ~v\n" my-thunk)
  (printf "exp is ~v\n" (exp-closure-exp my-thunk))
  (printf "result is ~v\n" (my-thunk)))

This produces

fun is #<procedure:...tmp/lambda.rkt:11:19>
exp is '(exp-lambda () (+ 1 2))
result is 3

A better version of this macro might propagate the source location of the macro use to the lambda expression it creates, or the inferred name (see syntax-local-infer-name), or both.