Preheat the oven to 220C, fan-forced. Wipe the mushrooms clean and cut into quarters. Finely chop the onion. Crush or finely chop the garlic. Strip the thyme leaves, discarding the tough stems.
Step 2 — Roast sweet potato
Line a large oven tray with baking paper. Cut the unpeeled sweet potatoes into 1cm chunks. Put the sweet potato on the lined tray, drizzle with 2 tsp olive oil, season with salt and pepper and toss to coat. Roast for 20 mins or until golden and tender.
Step 3 — Make stock
While the sweet potato is roasting, crumble the stock cubes into a medium saucepan, add the half the thyme and 1L (4 cups) water. Bring to the boil, then reduce the heat to low and keep at a gentle simmer.
Step 4 — Cook aromatics
Meanwhile, heat 1 tbs olive oil in a large deep frypan over medium-high heat. Cook the mushrooms, stirring, for 4 mins or until golden. Remove from pan. Heat 2 tsp olive oil in the pan. Reduce the heat to medium. Add the onion, season with salt and cook, stirring regularly, for 3 mins or until softened. Add the garlic and remaining thyme and cook, stirring, for 1 min.
Step 5 — Start risotto
Add the rice to the onion mixture and cook, stirring, for 1 min or until the grains are well coated. Add the hot stock, 1 cup at a time, to the pan, stirring continuously and allowing the stock to be absorbed before adding the next cup, until all stock is used, the rice is cooked and the mixture is thick (this will take about 25 mins). Rice should be creamy but still retain some bite.
Step 6 — Finish risotto
Shave one-quarter of the parmesan with a vegetable peeler, then finely grate the remaining parmesan. Stir the mushrooms, sweet potato, spinach and grated parmesan into the risotto and cook for 1 min or until the spinach is wilted. Taste, then season with salt and pepper. Divide the risotto among bowls and scatter over the shaved parmesan to serve.