For the potatoes: In a bowl smash the boiled potatoes. Add the sour cream, onions, parsley and egg yolk, and mix well. Season with salt and pepper. Form the mashed potato mixture into 4 cakes and coat with the breadcrumbs. Refrigerate and hold for later use.
For the Cornish hens: Sprinkle the Cornish hens on both sides with salt and pepper. Heat a large saute pan over high heat, add the grapeseed oil and heat until it just begins to smoke. Carefully lay the hens in the pan, skin-side down. Sear the hens until the skin is golden brown and releases from the pan. Reduce the heat to medium-high, turn the hens over, and cook for another 2 to 3 minutes.
Add the chicken stock and cover the pan with a lid. Continue to cook the hens until they are cooked through, or when an internal thermometer reaches 160 degrees F along the thigh bone. Remove the hens from the pan and let rest.
Add the demi glace and garlic to the stock in the pan and reduce by half.
Remove the pan from the heat and add the butter, and then whisk or swirl in the pan to incorporate. Add the tarragon and season the jus with salt and pepper.
Finish the potato cakes. In a saute pan over high heat, heat the grapeseed oil until it just begins to smoke. Place the potato cakes in the pan and cook until the cakes are golden brown. Reduce the heat to medium and flip the cakes and cook until both sides are golden brown. Remove the potato cakes and hold hot for plating.
For the broccoli rabe: In a saute pan, heat the grapeseed oil and add the broccoli rabe. Saute for 2 to 3 minutes. Season with salt and pepper and add the butter. Remove from the pan to a paper-towel-lined plate.
To plate, spoon 2 ounces of the tarragon jus on to the center of each plate. Place a portion of the broccoli rabe in the center of the sauce. Place a potato cake on top each broccoli rabe pile, then place either half or a whole hen over the potato cakes. Pour the remainder of the sauce over the hens.