1. Heat the stock in a saucepan and keep hot over low heat.
2. Heat the olive oil over medium heat in a separate medium-sized pot. Add the onion and cook for 2 to 3 minutes, until the onion turns translucent.
3. Add the rice to the onion mixture and stir.
4. Turn the heat to low, add about 1 cup of the hot stock to the rice mixture, and stir slowly until the stock is absorbed.
5. Continue to add the stock 1 cup at a time, stirring slowly, letting the rice absorb the stock before adding more.
6. The risotto is cooked when it is creamy on the outside and slightly firm (al dente) in the center. Stir in the Parmesan cheese and keep warm. Season with salt and pepper to taste. If the risotto is too thick, add a little more stock until it becomes creamy.
For the scallops and sauce:
1. In a large sauté pan over medium-high heat, add the mustard seeds, shaking the pan to keep them from burning. Once they begin to pop (like popcorn), lower the heat and add the olive oil and onions.
2. Cook the onions for about 5 minutes until they turn golden, then add the garlic and cook until it begins to soften. Add the tomatoes, 1/4 cup of the stock and, if desired, the Dijon mustard. Simmer for about 5 minutes for the flavors to blend.
3. Rinse the scallops and remove the thick, tough muscle on the sides of the scallops. Pat the scallops dry with a paper towel.
4. Add the scallops to the tomato sauce and turn up the heat to medium. Cook the scallops in the sauce for about 5 minutes.
5. Transfer the risotto to warm serving bowls and serve with the sauce.