Place radish and garlic 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 radish. Set aside to cool.
Make the Radicchio Slaw:
In a medium bowl, whisk together vinegar, sugar, olive oil and celery seed. Add sliced radicchio and season with salt and pepper. Refrigerate until ready to use.
Make the Chicken Burgers:
Preheat oven to 375 degrees and line a baking sheet with parchment paper.
In a large bowl, pour milk over bread crumbs and set aside to absorb. In a saute pan set over medium-low heat, cook bacon and onion until translucent, about 5 to 7 minutes. Add garlic and cook for one minute, then remove from heat.
Mix egg, ground chicken, pancetta, onion, garlic and parsley into the breadcrumbs, taking care not to overmix. Season with salt and pepper and form into 12 balls, lightly compressing to create a thick, burger shape. Bake on the prepared baking sheet 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 chicken patty on each square. Top with pickled radishes. Place the roll tops on top and brush with butter. Sprinkle with poppy seeds and bake for 7 to 9 minutes, until warmed through. Serve immediately.