How should I proceed? Feb 2023 my former boss left company. In July sent me check for 20k, this year sent me a 1099-NEC. I'm not self employed & want to avoid huge bill.

It depends. You can choose to treat this payment as wages, as an employee and report it using Form 4852, which is a substitute W-2.  Once completed three things will occur:

1. You will be charged only the employee portion of social security and medicare tax (not double as a self employed person).
2. The IRS will go back to the employer for the employer portion of those taxes.
3. You must mail your tax return to the IRS - TurboTax will provide the instructions and address.  
   • How do I file a substitute W-2 using Form 4852?
   • Keep all the documents in your file to show you were an employee 

This is definitely taxable income to you (personal income tax as well as employee share of social security and medicare tax).  The only other option would be to file as a self employed individual and you are not required to do that if you were an employee.