In a medium oven-proof pot with a tight fitting light, heat the olive oil over medium-high heat. Add the butternut squash, mushrooms, onion, garlic, salt and pepper. Cook the vegetables, stirring frequently, until they have started to soften and their volume is reduced by approximately half, about 5-7 minutes.
Stir in the champagne and cook, scrapping the bottom of the pot to loosen any browned pieces, until the liquid is almost completely evaporated, about 3-5 minutes. Add the Arborio rice, stirring to coat. Then add the vegetable broth. Bring everything to a boil, then cover the pot and transfer it to your pre-heated oven to bake for 20 minutes.
Remove the pot from the oven and carefully remove the lid. Stir in the spinach, Parmesan cheese, thyme leaves and lemon zest. There will be some liquid remaining, but it will get absorbed as the risotto rests.
Let the risotto rest for about 5 minutes before transferring it to serving dishes. Enjoy while warm.
Leftovers can be stored in an airtight container in your refrigerator for 2-3 days.