Start this recipe by removing the stalks from the mushrooms (reserve them for the stuffing). Drizzle the mushrooms with a tablespoon of olive oil and season with salt and freshly ground black pepper. Heat a griddle pan and fry the mushrooms, gill-side down for a couple of minutes or until they begin to turn golden-brown. Remove the mushrooms from the pan and put aside. Heat the remaining oil in the same frying pan and gently fry the onion and garlic for about five minutes, or until softened. Chop the reserved mushroom stalks finely and add them to the pan, along with the chopped pancetta and thyme. Fry for 4 minutes or until the bacon is cooked and browned. Mix in the chopped spinach and season well with salt and pepper. Remove off the heat and allow to cool down before mixing in the diced Brie cheese. Divide this mixture into 4, and use to fill the reserved Portobello mushrooms, piling the filling on. Place onto a baking sheet, sprinkle over the breadcrumbs and continue to cook for a further 6-8 minutes in the oven at 180`c. Remove the cooked mushrooms from the oven and serve immediately on large plates with a few mixed salad leaves.