Prepare the ham: If using leftover ham roast, trim off any excess fat. Finely dice the ham into very small pieces, or pulse it in a food processor until it reaches a finely ground consistency. If using deli ham, simply dice it finely.

Combine wet ingredients: In a medium mixing bowl, combine the mayonnaise, sweet pickle relish, and Dijon mustard. Stir until well incorporated.

Add aromatics and seasonings: Add the finely chopped celery and red onion to the mayonnaise mixture. Stir to combine.

Mix everything together: Add the prepared ham to the bowl with the wet ingredients and vegetables. Season with salt and black pepper.

Stir until thoroughly combined. Taste and adjust seasonings as needed. If the salad seems too thick, you can add a little more mayonnaise until it reaches your desired consistency.

Chill before serving: Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. This step is crucial for the best taste.

Serve: Garnish with fresh parsley if desired. Serve chilled.