Preheat broiler.
In a medium-size pot, add potatoes and cover with cold water. Bring up to a boil and cook until tender, about 10 minutes. Once the potatoes have finished cooking, drain them and put them back into the same pot they were boiled in.
While potatoes are cooking, place a large skillet over medium-high heat with 1 turn of the pan of EVOO, about 1 tablespoon. Add the chicken, season it with salt and pepper, and cook until golden brown and cooked through, 6-7 minutes.
While potatoes are cooking, place a large skillet over medium-high heat with 1 turn of the pan of EVOO, about 1 tablespoon. Add the chicken, season it with salt and pepper, and cook until golden brown and cooked through, 6-7 minutes.
Remove the chicken from the pan to a plate or bowl and toss the carrots, peas, scallions and tarragon into the pan. Season the veggies with a little salt and pepper, and cook until tender, about 2 minutes.
Scoot everything in the pan over to one side and melt in the butter. Sprinkle the flour over the butter and cook for about 1 minute. Whisk the white wine and chicken stock into the flour-butter mixture and cook until thickened, about 1 minute. Season the sauce with a pinch of salt and ground black pepper, then add the chicken back to the pan along with the Dijon mustard. Give everything a good stir and turn it out into a casserole dish.
When your potatoes are done cooking and drained, add the herb cheese, egg yolks, parsley, milk or chicken stock and some salt and pepper to the spuds. Mash them up until they're the consistency you like (you can add more liquid if you like them a little thinner), then spread them over top of the chicken and veggie filling in the casserole dish.
Put the dish under the broiler until the top is golden brown, about 2 minutes. Keep your eye on it so it doesn't burn!
When the casserole comes out of the oven, dust the top lightly with some paprika and serve it up with your favorite salad alongside.