Heat the olive oil in a large saucepan over medium heat. Add the chopped onion and garlic and sauté for 3-4 minutes, until softened.
Add the Arborio rice and stir for 1-2 minutes until it is coated with the oil and lightly toasted.
Pour in the dry white wine and stir until it is absorbed.
Add the vegetable broth, 1/2 cup at a time, stirring constantly and waiting for each addition to be absorbed before adding more.
After about 20 minutes, when the rice is almost cooked, stir in the saffron threads, lemon zest, lemon juice, and Parmesan cheese. Season with salt and pepper to taste.
In a separate pan, melt the butter over high heat. Add the scallops and sear for about 2-3 minutes on each side, until golden brown and cooked through.
Serve the risotto topped with the seared scallops.