In a medium saucepan, bring the vegetable broth to a gentle simmer. Keep it warm over low heat.
In a large, heavy-bottomed pot or skillet, heat the olive oil and butter over medium heat. Add the chopped onion and sauté for about 3-4 minutes, until it becomes translucent.
Stir in the minced garlic and cook for another 1-2 minutes until fragrant.
Add the Arborio rice to the pot and toast it for about 2 minutes, stirring constantly until the edges of the rice become slightly translucent.
Begin adding the warm vegetable broth to the rice, one ladle at a time, stirring frequently. Wait until the liquid is mostly absorbed before adding the next ladle of broth. Continue this process until the rice is al dente and creamy, about 18-20 minutes.
Once the risotto reaches the desired consistency, remove the pot from heat. Stir in the grated Parmesan cheese, lemon zest, and lemon juice. Mix well until creamy and smooth.
Season with salt and pepper to taste.
Serve the risotto hot, garnished with fresh parsley or thyme for an added touch of flavor and color.