Preheat oven to 200°C (400°F). Wrap beetroot in foil and roast for 30-40 minutes until tender. Cool, peel, and dice.
Heat olive oil in a large heavy-bottomed pan over medium heat. Add chopped onion and sauté for 3-4 minutes until softened.
Add minced garlic and cook for another minute until fragrant.
Add risotto rice to the pan and stir for 2-3 minutes until grains are well-coated with oil and slightly translucent.
Pour in white wine and stir continuously until completely absorbed.
Add hot vegetable stock one ladle at a time, stirring continuously. Wait until each addition is absorbed before adding more.
After 18-20 minutes, when rice is creamy but still has a slight bite, fold in the diced beetroot.
Season with salt, pepper, and lemon juice if using. Serve immediately with grated Parmesan if desired.