Marinate the quail in olive oil, thyme, and sage for at least 2 hours, up to overnight, in the refrigerator.
Preheat a grill. Preheat the oven to 400 degrees F.
Remove the quail from the marinade and season with salt and pepper. Place the quail, breast-side-up, on a medium hot grill. Cook for 3 minutes, then turn over so the breast-side is down, and cook for 2 minutes. Then lift and adjust position on the grill to create an "x" mark on the breast. Finish cooking for about 4 minutes, about 8 to 9 minutes total cooking time.
While the quail are cooking, sear the foie gras in a dry, medium-hot pan. Place in the oven for 4 minutes. Remove from the oven, place the foie gras on a cutting board and slice into 8 pieces. Lightly toast the brioche, place a piece of foie gras on each half and a quail on top. Spoon the sauce over the quail and garnish with Sweet Potato Shoestrings.
Rose Geranium Sauce:
Place the shallots, wine, and vinegar in a small pot. Bring to a boil, the reduce the heat and simmer until reduced by half. Add the stock and continue to reduce until you have about 1/2 cup of liquid left. Whisk in the jelly, then add the butter and season, to taste. The sauce should be glossy and syrupy, but not too sweet. Add a drop of vinegar, to balance, if necessary. Set aside and keep warm.
Sweet Potato Brioche:
Bake the sweet potatoes until very soft. Let cool. Peel and weigh out 12 ounces. Put the sweet potatoes in the bowl of an electric mixer. Using a paddle, mix for 2 minutes on medium speed or until mashed. Then add egg, milk, and yeast and mix for 1 minute. Add the rest of the dry ingredients and mix for 6 minutes on medium speed. Let rest for 15 minutes.
Preheat the oven to 325 degrees F. Add the cold butter in stages on speed 2. Mix for 6 to 8 minutes to smooth out the brioche, until it does not stick to the sides of the bowl. Divide into 2-ounce balls and place in muffin tins or brioche molds. Let rest in a warm place for 1 1/2 hours. Brush with egg wash and bake until golden brown, about 10 minutes.
Sweet Potato Shoestrings:
Preheat a fryer to 325 degrees F. Add the potatoes and fry until crispy. Drain on paper towels and season with salt.