Place an oven-safe Dutch oven on the stove-top over medium heat. Add butter and melt. Add onion and cook, stirring, until softened. Add rice and cook, stirring, for a minute or so. Add wine and allow to cook off for about 2 minutes, stirring occasionally. Add heated stock and stir to combine. Cover with lid (or tightly with aluminum foil) and place into oven. Bake for about 45 minutes, or until most (but not quite all) of the liquid has been absorbed.

Meanwhile, place a skillet over medium-high heat. Add butter and melt. Add 2/3 of the mushrooms and a light sprinkling of salt and cook, stirring, for several minutes until much of their liquid has cooked off. Add the final 1/3 of the mushrooms and cook, stirring, until softened and deep golden. Add thyme leaves and stir to combine. Add marsala and allow to simmer for about 2 minutes, to allow the alcohol to cook off. add the broth and stir to combine. In a small bowl, stir together the cornstarch and cream. Add to the skillet and cook, stirring regularly, until sauce is warmed and thickened. Taste and season with salt and pepper, as needed. (Turn of heat under mushrooms if necessary and re-heat when risotto is done, if done well before the risotto).

When risotto have baked for 45 minutes, remove from oven. Stir in Parmesan cheese and frozen peas and stir well for a minute to combine. Cover with lid and allow to stand for a couple of minutes. Remove lid. Taste and season with salt and pepper, as needed. Add a splash more broth, if risotto is too thick.

To serve, spoon risotto into the bottom of a shallow bowl. Spoon warm Marsala Mushrooms over-top. Garnish with freshly grated Parmesan cheese and some sprigs of thyme, if desired.