Heat the seafood stock in a saucepan over low heat and keep it warm throughout the cooking process.
In a large saucepan, heat the olive oil over medium heat. Add the chopped onion and sauté for about 5 minutes until translucent.
Stir in the minced garlic and cook for another minute until fragrant.
Add the Arborio rice to the pan and cook for 2-3 minutes, stirring constantly until the rice is lightly toasted and coated in oil.
Pour in the white wine and cook until it's mostly absorbed by the rice, stirring frequently.
Begin adding the warm seafood stock, one ladle at a time, stirring continuously. Allow each addition to be absorbed before adding the next. This process should take about 15-20 minutes.
When the rice is al dente, add the shrimp and mussels. Stir well and cook for another 5 minutes until the shrimp are pink and the mussels have opened.
Mix in the halved cherry tomatoes, chopped parsley, and grated Parmesan cheese. Stir until combined and the cheese is melted. Season with salt and black pepper to taste.
Remove from heat and let the risotto sit for a minute before serving. Serve warm, garnished with additional parsley and lemon wedges if desired.