Preheat oven to 350°F (175°C).
Line a baking sheet with parchment.
Slice Hawaiian rolls in half horizontally.
Place bottom halves on the sheet.
Layer cheddar cheese over bottom rolls.
Add mozzarella cheese on top.
Optionally, add ham slices.
Spread Dijon mustard if using.
Place top halves over cheese layers.
Mix melted butter, garlic, and onion powder.
Brush butter mixture over top rolls.
Cover rolls with foil.
Bake for 15 minutes.
Remove foil and bake five more minutes.
Cheese should be melted and rolls golden.
Garnish with chopped parsley.
Slice into individual mini sandwiches.
Serve warm immediately.