Pat the meat dry after chopping and before cooking.
Heat a Dutch oven or other heavy pot over medium-high heat. Add the oil and bacon and cook to crisp. Remove to a plate and drain all but 3-4 tablespoons of the drippings from the pot. Add the beef or lamb; season with salt and pepper and cook in two batches to brown and crust. Remove the meat to the plate with bacon.
To the meat drippings, add the onions, carrots, celery, garlic, bay herb bundle, salt and pepper. Cook, partially covered, for 10-12 minutes to soften, stirring occasionally. Add the flour and stir 1 minute to cook out the raw flour taste; add the Worcestershire sauce and wine and stir. Add the stock and beef or lamb and the bacon back to the pan; reduce the heat to simmer and cook, covered, until tender, stirring occasionally, up to 40 minutes.
Place the potatoes in a pot, cover with water and bring to a boil. Salt the water and cook to tender; drain and pass through a ricer. Stir in the warm milk to thin to your liking, then add the sour cream, horseradish, cheeses, pepper, nutmeg and chives. Adjust the seasoning, then stir in the egg yolks and pipe or spread over the meat filling in casserole or single-serve dishes.
Heat the broiler and place a rack in the center of the oven. Brown the potatoes and sprinkle with paprika to serve.