Tip a few tablespoons of the flour on to a large plate and roll the chicken thighs in the flour so that they are nicely coated.
Heat the olive oil in large, heavy-based casserole pan over a medium heat. When hot, add the floured thighs skin-side down and cook them until the skin has a golden colour (about 4 minutes). Turn the thighs over and seal the other side for about 2 minutes. Then, remove the thighs from the pan and leave them to one side.
Wipe the dish clean with some kitchen paper then add the pancetta. Cook it for 2 minutes to release the fat, then add the garlic, thyme and portobello mushrooms and cook for a further 5 minutes.
Meanwhile, drain the porcini, reserving the soaking water, and finely chop them. Add the chopped porcini to the pan with the soaking water, along with the leeks, and cook for a further 5 minutes to soften.
Stir through the marsala and the mascarpone, then season with salt and pepper.
Place the chicken thighs back into the casserole, skin-side up, on top of the other ingredients. Cover the pan with a lid and transfer it to the oven. Bake the thighs for 30 minutes, then remove the lid and cook for a further 10 minutes, until the chicken is cooked through and the skin is crisp.
Serve with creamy wet polenta, flavoured with butter and parmesan, or buttery mashed potatoes.