How to Make It
Preheat the oven to 425°. Season the hens all over with salt and pepper and arrange them on 2 large rimmed baking sheets, skin side up. Drizzle with 1/4 cup of the oil and scatter the rosemary all around. Roast the hens in the upper and middle thirds of the oven for 40 minutes, until cooked through, shifting the pans from top to bottom and front to back halfway through. Pour the cooking juices into a gravy boat.
Meanwhile, in a large skillet, heat the remaining 1/4 cup of olive oil until shimmering. Arrange 4 slices of the prosciutto in the skillet in a slightly overlapping layer and cook over moderately high heat, turning once, until crisp, about 3 minutes. Transfer to a paper towel–lined plate. Repeat with the remaining prosciutto.
Pour off all but 2 tablespoons of the fat from the skillet and add the butter. Add the endives, cut sides down, and cook over moderately high heat, turning until browned and tender, 12 minutes. Season with salt and pepper. Add the greens to the pan and cook, stirring, just until wilted, 2 minutes.
Transfer the hens to platters and surround with the endives, greens and crispy prosciutto. Serve with boiled potatoes, passing the reserved cooking juices at the table.
You can ask your butcher to quarter the guinea hens for you. Chicken can be substituted for guinea hens, but it will need to cook for about 15 minutes longer.