I've made these a few times now with different variations.  They are really good!  And extra yummy the second day.  Here's what I learned:

1. I like to make 6 ham and 6 turkey.
2. I prefer the flavor of the prepared yellow mustard over the dry mustard.  Gets a little clumpy but worth it for flavor.
3. Using a pastry brush to apply the sauce is the easiest way to go.
   
4. Don't forget to cover them so the cheese melts and bread stays soft!  I have made this mistake.
5. Recently at a family gathering we had another version with brown sugar that was delicious!  You can probably do an Internet search to find the recipe with that variation.  Definitely more sweet than tangy.

 
Ham and Cheese Sliders

12 white dinner rolls
12 pieces ham, turkey, or both
12 small slices Swiss cheese
1/3 cup mayonnaise

Poppy seed sauce
1/2 tablespoon poppy seeds
2 teaspoons prepared yellow mustard or 1 teaspoon dry mustard
1/4 cup butter, melted
1/2 tablespoon dehydrated minced onion
1/4 teaspoon Worcestershire sauce

Spread mayonnaise on both sides of each roll. Place a slice of ham or turkey and a slice of Swiss inside of each roll. Close rolls and place them close together in a 9X13 baking dish.

In a medium bowl, whisk together all of the poppy seed sauce ingredients. Using a pastry brush, brush the sauce generously over all of the sandwiches. Let sit 10 minutes or until butter sets slightly. You can also make this the day before and let them hang out in the fridge overnight (covered of course).

Cover with foil and bake at 350 degrees for 12-15 minutes or until cheese is melted. Uncover and cook for 2 additional minutes or until tops are slightly brown and crispy. Serve warm.