Set your oven to 350°F (175°C) with a rack placed in the lower third of the oven. As it preheats, bring 1-inch (2.5 cm) of water to a boil in a medium skillet. Add the pearl onions and cook for 1-2 minutes. Drain the onions, then trim the ends and peel each one. Transfer them to a plate and set aside.
Slice carrots into 2-inch (5 cm) logs. Heat 1 tablespoon of oil in a large ovenproof pot (I use a 4-quart Staub). Add the carrots in a single layer, sprinkle with a pinch of salt and cook over medium-high heat for about 10-15 minutes, turning them frequently until they’re golden brown on every side. (You can do this in batches to avoid crowding the pot.)
Transfer carrots to a plate, add another tablespoon of oil to the pot. Toss in the Brussels sprouts and cook for 2-3 minutes, stirring occasionally, until lightly browned. Add the pearl onions and, stirring occasionally, cook for another 2-3 minutes.
Return the carrots back to the pot and pour in the wine. Simmer for 3-5 minutes, then stir in the porcini powder and vegetable stock. You want the stock to only partially cover the vegetables. Sprinkle vegetables with a large pinch of salt and a pinch of pepper, cover pot with lid and transfer to the oven.
Braise vegetables for about 30-40 minutes. Check on them after the first 20 minutes pass, turn them over and add more liquid if needed.
The osso bucco is ready once all the vegetables are tender and easily pierced with a thin knife. Remove pot from the oven and taste the sauce, add more salt and pepper if desired. Keep osso buco covered until serving time, it stays warm for 1 hour easily.
To serve: Spoon some mashed potatoes (or mashed cauliflower or polenta) into a shallow bowl, top with vegetables from osso buco, then add a few tablespoons of the sauce. Sprinkle with parsley and serve. Store leftover osso buco covered in the refrigerator, reheat slowly on the stove.