Preheat the oven to 375 F.
In a large soup pot, bring 12 cups of water to a boil. Add the pasta noodles and cook for 7-8 minutes, stirring occasionally, until the pasta is al dente. Be careful not to overcook. The pasta will get cooked again in the oven and it will get mushy if it's too soft. Drain and rinse the pasta through a colander. Transfer to a large bowl and stir in the vegan cheese sauce until the noodles are completely coated and set aside.
Thinly slice the onion. Heat 1 tablespoon of olive oil in a large skillet over medium-low heat. Cook the onions slowly, stirring them every few minutes. After 10 minutes, sprinkle with a pinch of salt and continue to cook for another 5-10 minutes until onions have reduced in size and are caramel in color. Remove from heat.
Meanwhile cut the Brussels sprouts into half. If the sprouts are very large, you can quarter them. In a second skillet, heat up the remaining tablespoon of oil over medium heat. Add the Brussels sprouts and sauté for 10 minutes, until they are slightly soft and blackened around the edges.
Add the caramelized onions and sautéed Brussels sprouts to the prepared pasta, stirring it all together. Transfer the pasta dish to a 2-quart casserole dish, tightly pressing it down. Top with bread crumbs and bake for 20-25 minutes, until the bread crumbs are golden brown. Remove from the oven and serve.