Blanch the Cabbage

Bring a large pot of salted water to a rolling boil. Cut into the cabbage at the base to remove the stem and core. Gently plunge the cabbage into the salted boiling water and simmer 8–10 minutes, until the leaves are softened and translucent. Remove with a slotted spoon and let cool enough to handle.

Prepare the Filling

In a large skillet, heat the olive oil and butter until the butter melts. Add chopped onions and sauté ~8 minutes until translucent and soft. Add the ground turkey and cook, turning, until it changes color and is mostly cooked through. Stir in the rice, coating it in the oil and combining thoroughly with the meat. Add chicken stock, reduce heat to low, and simmer until the rice absorbs most of the liquid. Season with salt and pepper. Remove from heat and stir in the parsley and dill. Set aside to cool slightly.

Roll the Cabbage

When both the cabbage leaves and filling are cool enough, gently separate the leaves. If leaves are large, you may halve them. Trim the tough central ribs so the leaves roll more easily. Oil the bottom of a large, wide pot. Lay torn or too-small leaves (from trimming) on the bottom of the pot as a base layer. Place about 2 teaspoons to 1 tablespoon of the filling (depending on leaf size) onto each leaf. Fold in the edges and roll into a cylinder, placing seam side down in the pot. Pack snugly; if necessary, create a second layer.

Cook the Rolls

Pour enough water or stock into the pot to cover the rolls by about an inch. Place an inverted plate on top of the rolls to keep them submerged. Bring to a simmer over medium heat, then reduce to low and simmer gently for about 1 ½ hours, or until the cabbage is very tender and the rice is fully cooked. You should aim to end up with about 1½–2 cups of cooking liquid remaining—this will be used for the sauce.

Make the Avgolemono Cream Sauce

Whisk the egg yolks until very frothy then whisk in the lemon juice. Take a ladleful of hot cooking liquid from the pot and add it drop by drop, whisking constantly, into the egg-lemon mixture (this tempers the eggs). Add a second ladleful and any remaining pot juices using the same method. In a medium saucepan, melt the butter over medium heat and whisk in the flour. Whisk vigorously ~3–4 minutes. Slowly whisk in the heavy cream, stirring until warm. Reduce heat and gradually whisk in the tempered egg-lemon mixture into the cream mixture, stirring constantly. Season with salt & pepper. Immediately pour the sauce over the cabbage rolls in the pot, tilt the pot so the sauce distributes, and serve immediately.