1. Set your oven to 400°F (200°C).
2. Add potatoes to a large stockpot or Dutch oven. Cover the potatoes with water by 2 inches. Put the lid on the pot and bring it to a boil. Once boiling, reduce heat to medium-high, slightly lift the lid, and cook until the potatoes are fork-tender. This will take approximately 20-25 minutes.
3. While the potatoes cook, peel as many outer layers as you can off the head of garlic while keeping it intact. Cut off the top third of the head of garlic to reveal the cloves. Place the garlic on heavy-duty foil or two layers of regular foil. Drizzle with olive oil, season with kosher salt, and black pepper. Fold up the edges to make a pouch. Place it on the middle oven rack and roast for 15 minutes.
4. Cut the trimmed leeks in half, leaving the root end intact but trimming off the roots lengthwise. Rinse the sandy dirt away under a steady stream of water. Pat dry and place on a lined, rimmed baking sheet. Drizzle with oil and season with kosher salt and black pepper.
5. While the potatoes are cooking and the leeks and garlic are roasting, add bacon to a skillet and cook until it's crispy over medium-low heat. Remove all but 2 tablespoons of the bacon fat once it's crispy. Add the chopped kale, stir, and cook until it wilts.
6. When the timer for the garlic goes off, move the pouch to the tray with the leeks and roast for another 20-22 minutes or until the leeks caramelize.
7. Move the caramelized leeks to a food processor and use tongs to carefully squeeze the cloves out of the garlic's head. Secure the lid and blend the leeks and garlic until they resemble a paste.
8. When the potatoes are tender, drain the water (I use the lid and keep the potatoes in the pot). Add milk, sour cream, and butter, then mash the potatoes. Season with 2 teaspoons of kosher salt (more or less to your taste) and some freshly ground black pepper.
9. Stir in the roasted leek and garlic paste before combining the roasted bacon and sautéed kale.
10. Taste and adjust the seasoning with more salt and pepper as needed.
11. Transfer to a bowl and shape with a spoon. Top with a little melted butter (optional), more freshly ground black pepper, and sprinkle with sliced chives.