Preheat oven to 350°F.
With a small knife, do incisions in the roast (distribute them well) and insert garlic. In a casserole dish, melt butter in oil, brown the roast on all sides, 5 to 7 minutes total. Remove the roast from the casserole.
Combine Dijon mustard, maple syrup and herbs of Provence. Baste generously the roast with this mixture and set aside the leftover for the stock. Sprinkle with garlic salt, salt and pepper. Cover the bottom of the casserole with onions, and put the roast on top and wet with the chicken stock. You can pour the leftover of the mixture of mustard and maple syrup on top.
Cover and cook in the oven for 90 minutes.
Remove the roast from the oven, cover with foil paper and let rest 10 minutes before serving. Set aside the onions in the serving dish.
Meanwhile, put the cooking juice in a small saucepan, add the cream, whisk a little bit, adjust seasoning and let reduce to medium heat 5 minutes. Incorporate the kneaded butter, whisk energetically and let thicken 3 or 4 minutes, until smooth.
Serve the roast in slices with the sauce on top along with the onions.