Heat a large sauce pot over medium high heat. Add enough olive oil to coat the bottom of the pot. Add the shallots and garlic and cook until the shallots start to turn translucent. Add the ground pepper and toast for 30 seconds more.
Add the Cognac to deglaze the pan.
Add the roasted mushrooms, porcini jus, heavy cream and dijon mustard. Stir to combine and allow the jus to come to a boil, then reduce the heat to a simmer and cook for 3-5 minutes.
Remove from heat and stir in the butter. Season with salt to taste. Serve over mashed potatoes and seasonal vegetables, like haricots verts, baby carrots, or snow peas.