Bring the stock to a boil in a saucepan. Meanwhile, heat the butter and oil in a large pan. Add the shallots, thyme and garlic. Season, then cook for 5 minutes. Add the barley and stir for 1 minute, then pour in the Marsala or wine and stir until absorbed.
Add 1 ladle of stock to the barley mix; stir until absorbed. Tip in the mushrooms. Add another ladle of stock; again, stir until absorbed. Repeat, a ladleful at a time, stirring constantly, until thickened.
Taste the barley after 20-25 minutes and, when al dente, add one more ladle of stock and the peas. Cook for 2-3 minutes to warm the peas through, then stir in half the cheese. Serve the risotto with the remaining cheese on the side to sprinkle over.
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