Bring the vegetable stock to a boil in a large saucepan over medium-high heat.
Slowly add the polenta in a steady stream, whisking as you go to prevent any clumps. Whisk in the salt. When the polenta begins to bubble, reduce the heat to medium-low, and continue to whisk to prevent clumps. Cook for 10-20 minutes, until the mixture is thick and pulls away from the pot.
Lightly oil a 9”x12” baking or casserole dish. Pour the polenta into the dish, using a rubber spatula to even out the mixture.
Allow the polenta to cool and then place in the refrigerator for 2 hours or overnight.
Line a roasting pan with parchment paper and preheat the oven to 400°.
Remove the polenta from the pan and cut into 1-inch cubes. Spread the cubes evenly on the roasting pan, making sure the pieces don’t overlap, and bake for 20-30 minutes, flipping halfway through, until the cubes are slightly brown and crispy.
While the polenta is baking, heat the oil in a large pan over medium high heat, and add the shallots, mushroom, and celery along with a pinch of salt. Cook, stirring frequently, until the vegetables are fragrant and slightly browned.
Add the spinach and fresh herbs, and cook until the spinach has wilted, about 2 or 3 more minutes.
Combine the polenta and vegetables in a casserole dish, adding the rest of the salt and the black pepper. Carefully fold ¼ cup vegetable stock into the mixture. If it seems too dry, add another ¼ cup of stock.
Cook for 30-40 minutes, until lightly browned and heated throughout.