Preheat oven to 375 degrees F.
Heat a large nonstick skillet over medium high heat. Add a drizzle of oil and the portobello caps. Season caps with salt and pepper, to taste, and cook 3 minutes on each side. Add balsamic to the pan and allow the vinegar to cook away as it coats the caps. Transfer balsamic glazed caps to a cookie sheet. Return pan to the stove and add oil, garlic and onion. Saute onions and garlic 3 minutes, add spinach to the pan and let it wilt. Coarsely chop artichoke hearts in the food processor and add to the spinach. Season veggies with salt and pepper and thyme. Add chopped toast and dampen stuffing with chicken or vegetable stock. Combine stuffing and sprinkle in a little grated cheese. Top each mushroom with 1/5 of the filling. Set mushrooms in oven for 5 minutes to set the filling. Cut each mushroom into 4 pieces and transfer to a serving dish.