Mockery mock and spy called 0 times on custom class in controller

I'm having difficulty with the spy and mock in Laravel 7 test when I test for MyCustomClass.

I have tried both mock before running $this->get and spy after $this->get. Both with the same error message (*below).

When running debug in the controller the $myCustomClass is still the MyCustomClass and not the mocked object.

MyCustomClass

class MyCustomClass
{
 public function execute()
 {
   return 'hello';
 }

MyController

class MyController
{
 public function show()
 {
   $myCustomClass = new MyCustomClass();

   $data = $myCustomClass->execute();

   return $data;
 }

private $mySpy;

public function testGetATreeNoCache()
    {
        $spy = $this->spy(MyCustomClass::class); 

        $response = $this->get('/my/path');

        $spy->shouldHaveReceived('execute');

        $response->assertStatus(200);
    }

Error

Method execute(<Any Arguments>) from Mockery_2_App_MyCustomClass should be called
 at least 1 times but called 0 times.