Preheat your oven to broil.
Split six slider buns in half and place them open-faced on a baking sheet lined with Parchment paper. Toast until lightly brown, which happens very fast - so watch them close!
Mix the spicy mustard, sour cream, and mayonnaise with a pinch of salt and pepper. Spread each half of the toasted buns with mustard sauce.
Top the bottom half of the buns with 2 squares of cheese followed by one slice each turkey and ham.
Add some onions and top with two more squares of cheese.
Remove the top piece of the bun from the pan and broil the sliders until the cheese is melted - about 40 seconds.
Add the top of the buns and serve immediately!