Stuffed Poblano Chiles with Avocado and Potatoes

1. Put the potatoes in a medium saucepan and add lightly salted water to cover. Bring to a boil over high heat. Reduce the heat to medium-low and cook until the potatoes are tender about 30 minutes. Drain and let stand until cool enough to handle. Peel the potatoes and place in a medium bowl. Mash the potatoes, leaving them a bit lumpy
2. Make a lengthwise cut in each chile and remove the seeds. Combine the mashed potatoes, avocados, olive oil, and ground peppercorns in a medium bowl and season with the salt. Stuff the chiles with the potato mixture. Place in a 9-by-13-inch baking dish. Cover with plastic wrap and refrigerate.
3. To roast the garlic: Position a rack in the center of the oven and preheat the oven to 350°F. Bring the garlic and milk to a boil in a medium saucepan over medium heat, then boil for 3 minutes, taking care that the milk does not boil over. Drain the garlic in a sieve, discarding the milk. In a small bowl, toss the garlic cloves in the olive oil. Spread the garlic in a single layer on a baking sheet and cover with aluminum foil. Roast until tender and golden, about 20 minutes. Set aside.
4. To make the marinade: Bring the water and piloncillo or brown sugar to a boil in a small saucepan over high heat. Boil, stirring often, until the sugar is dissolved and the water is reduced to 1 cup, about 5 minutes. Remove from the heat.
5. Heat the oil in a medium skillet over medium heat. Add the onion and cook until it begins to soften, about 1 minute. Add the sugar water, vinegar, oregano, peppercorns, cloves, allspice, and bay leaves and bring to a simmer (do not boil). Remove from the heat, add the roasted garlic, and let cool completely. Pour the marinade over the chiles, cover, and refrigerate for at least 6 hours or up to 12 hours.
6. To serve, remove the chiles from the refrigerator and let come to room temperature. Place a single chile on each plate, along with some of the marinade, including some of the onion and garlic cloves.