Preheat the oven to 425 to 450 degrees. Sprinkle the birds with salt, pepper, and ½ teaspoon each paprika and cinnamon. Place on a rack in a shallow roasting pan and roast until the juices run clear when a thigh is pierced with a skewer, about 45 minutes. Let stand until cool enough to handle, and then cut into quarters, discarding the backbones. Place the poussin or hen quarters in a pan, cover with foil, and keep warm in a low oven, or set in a warm place.
Melt the butter in a medium saucepan over medium heat. Add the onions, and cook until soft. 8 to 10 minutes. Add the remaining 1 teaspoon paprika and 2 teaspoons cinnamon and cook 3 to 5 minutes. Add the stock and marsala and cook over high heat for a few minutes until the sauce is reduced and thickened. Add the cherries and walnuts, if using, and heat through. (The sauce and poussins or hens may be prepared ahead up to this point.) Remove the sauce from the heat (or, if made ahead, rewarm poussins or hens in the sauce and then remove from heat) and swirl in the sour cream. Season to taste with salt and pepper. Pour the sauce over the warm poussins or hens on a serving platter. Serve with rice and wild rice pilaf or cracked wheat and sautéed spinach. (Instead of using the optional walnuts in the sauce, toss them in the sautéed spinach if you’d like some crunch.)