While the potatoes are cooking, begin the stew. Place the dried porcinis and vegetable stock in a medium saucepan, bring to a boil, then remove from the heat and cover until needed.
Meanwhile, heat the olive oil in a large, deep sauté pan or Dutch oven over medium-high heat. Add the onion and a pinch of salt, and sauté until translucent, about 7 to 10 minutes. Add in the garlic, carrots, rutabaga, celeriac, and mushrooms, and cook until the mushrooms have released their liquid and the other vegetables are beginning to soften, about 5 minutes more.
While the vegetables are cooking, strain the rehydrated porcinis from the mushroom stock, reserving the stock, and roughly chop the mushrooms. Add the porcinis, thyme, and tomato paste to the vegetables, and cook for an additional minute or two.
Pour in the wine and allow it to reduce slightly, then add the reserved mushroom stock and salt. Bring to a boil, cover the pot, then reduce the heat and allow it to simmer for 20 minutes. Stir in the cooked lentils.
Combine the flour and water in a small bowl, and whisk together until smooth. Pour into the stew, stirring well, and continue to stir until the stew is slightly thickened, which will take approximately 5 minutes. Taste the stew and adjust seasonings as necessary.