Remove all fat from the lamb top rounds and discard. Combine rosemary, thyme, garlic, and 1/8 cup canola oil. Marinate lamb in herb-garlic mixture, covered and refrigerated, overnight.
For the Lamb Fennel Essence: Trim lamb scraps and dice into 1/8-inch cubes. Heat canola oil until just smoking. Pan fry lamb in batches. Cook until lamb is evenly caramelized dark brown, but not burnt. Strain off fat and de-glaze pan with vinegar and scrape fond from bottom of pan. Add thyme, fennel seeds, and red wine to pot. Reduce. Add demi-glace and simmer very slowly for 2 to 3 hours.
Add water to bring sauce up to original volume as needed. Reduce by half and season with salt. Strain through fine chinois. Adjust seasoning.
To finish, bring one cup of sauce to boil. Whip in two tablespoons of cold butter and check seasonings.
For The Lamb: Preheat oven to 450 degrees F.
Remove lamb from marinade. Season well with salt. Sear the lamb over high heat with one tablespoon oil in ovenproof pan. Sear to a dark brown, then turn lamb over, and transfer to the oven for 7 to 10 minutes, for medium rare. Remove from oven, cover with foil, and allow to rest for 5 minutes before slicing.
Place potatoes and salt in a large pot. Add cold water and cover. Bring to a boil. Reduce to a simmer and cook until completely tender, about 30 minutes. Drain well in a colander.
Combine 4 tablespoons butter and cream in a saucepan over low heat and heat until butter is melted.
Peel onions and slice thin as possible. To very hot skillet, add 2 tablespoons butter, then onions, and salt. Stir and continue to cook over high heat until they color. Then reduce heat and caramelize to a chestnut brown color.
Mash the cooked potatoes into the pot they were cooked in. Keep potatoes warm over a very low flame, stirring in the cream and butter and onion mixture. Taste and adjust the seasonings.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.