Combine Panko breadcrumbs and milk in a large mixing bowl.
Let sit for 5 minutes to allow the breadcrumbs to soak up the liquid.
Set a small sheet pan, baking dish, or cutting board next to the bowl (this is where you will line up the meatballs as you shape them).
To the breadcrumbs / milk, add turkey, 2 Tbsp onion, 1 ½ tsp Italian Herb Seasoning, and egg. Add salt (1 tsp if your Italian Seasoning doesn't include salt; 1/2 tsp if your Italian Seasoning does include salt).
Using your hands, gently combine all of the ingredients, mixing them just until combined (but being careful not to over-mix).
Use a 1 Tbsp measuring spoon to scoop up one heaping spoonful of the meatball mixture at a time and then roll it into a ball. (Don’t worry if they aren’t all perfect balls, but try to make them uniform in size so they cook evenly.)
Line up the meatballs on the prepared pan as you make them. (At this point, you can cover and refrigerate the meatballs to cook them later if you’d like.