Place carrots and dill in a heatproof jar. In a medium sauce pot combine water, vinegar, sugar and salt and bring to a boil. Allow to cool slightly, then pour over the carrots. Set aside to cool.
Make the Harissa Mayo:
Whisk together all the ingredients for the mayo in a small bowl and refrigerate until ready to use.
Make the Burgers:
Preheat oven to 375 degrees and line a baking sheet with parchment paper.
In a medium bowl, combine all ingredients, taking care not to overmix. Form into 12 patties, place on prepared baking sheet and bake for 20 to 25 minutes.
Assemble the Sliders:
Place a new sheet of parchment paper on the baking sheet.
Slice the sweet Hawaiian rolls in half lengthwise. Remove the tops and set aside. Place the bottom half on the prepared baking sheet. Spread radicchio slaw all over and place a burger on each square. Top with pickled carrots. Place the roll tops on top and brush with butter. Sprinkle with dried onion and bake for 7 to 9 minutes, until warmed through. Serve immediately.