Heat a large roasting pan or Dutch oven and add 3 tablespoons of the olive oil. Spread the potatoes in the roasting pan in a single layer and sprinkle with salt and pepper. Add the garlic. Saute the potatoes until golden and tender, 10 to 15 minutes. While the potatoes are browning, cut the bacon into 1/2-inch-wide pieces to make 'lardons." Put the bacon and the remaining tablespoon olive oil in a large skillet and cook over medium heat, stirring occasionally, until the bacon is just golden brown. Wash the greens. Cut off the bitter bottom stalks. Cut the leaves into 3-inch lengths. Separate the chive blossoms, if using, into individual florets.
When the potatoes are done, add the hot bacon to the potatoes using a slotted spoon, reserving the bacon fat in the pan. Add the greens to the reserved fat in the pan; add the sherry vinegar and toss to coat. Transfer the greens to the pan with the potatoes and toss to combine until the greens just wilt. Divide the salad among serving plates and garnish with the chive blossoms if using. Serve immediately.