In a large, deep skillet, cook the bacon slices over moderate heat until crisp, about 6 minutes. Using a slotted spoon, transfer the bacon to a plate. Pour off all but 1 tablespoon of the bacon fat in the skillet; reserve the fat in a small bowl.

Add the onions and garlic to the skillet and cook over moderate heat until lightly browned, about 3 minutes. Add the potatoes, carrots, sunchokes, and bay leaves and cook, stirring, for 3 minutes. Season with salt and pepper, add the brandy, and cook over high heat for 2 minutes. Add the wine; simmer for 5 minutes. Add the stock and simmer over moderate heat until the vegetables are tender, about 15 minutes.

Transfer the vegetables to a bowl. Discard the bay leaves. Boil the cooking liquid over high heat until reduced to 3/4 cup, about 12 minutes. Return the vegetables to the skillet and season with salt and pepper.

Preheat the oven to 400 degrees. Heat 1 tablespoon of the reserved bacon fat in each of 2 large nonstick skillets. Flatten the guinea hens by pressing down on the breast bones. Arrange the legs so that the thighs cover part of the breasts. Season the hens on both sides with salt and pepper. Add 1 hen to each skillet, skin side down, and cook over moderately high heat until browned, 5 minutes. Turn the hens and cook for 2 more minutes. Transfer the guinea hens to a large rimmed baking sheet, skin side up. Wipe out 1 skillet and repeat with 1 tablespoon of reserved bacon fat and the third hen. Set the hen on a second baking sheet. Tuck a rosemary sprig under each hen.

Roast the hens for about 35 minutes, switching the pans halfway through cooking, until an instant-read thermometer inserted in the inner thigh registers 160 degrees. Transfer the hens to a carving board and let rest for 10 minutes.

Add the bacon to the vegetables and reheat gently. Spread the watercress on a platter. Carve the hens and arrange the pieces on the watercress. Garnish with the lemon wedges and radishes and serve with the braised vegetables.