Sometimes the easiest solutions stare us in the face and it's easy to overlook them. I get asked this regularly and the easiest way to do this is to just set up Apple Mail with your email account and then change the default email application to Outlook.
You don't have to use Apple Mail again after that. So, while you're technically still adding an account to Mail, you don't have to use it, or you can remove it after you've set the default to Outlook.
EDIT
I just wanted to add that in some cases, certain Apple software (e.g. Photos) have their own settings for which email client to use when sharing content. So you will need to select Outlook as the default within these Apps as well if you want to share content via email.