Heat olive oil in a large pot over medium-high heat.
Sauté garlic and Brussels sprouts for 5 minutes.
Season with salt, pepper, and 1 tablespoon of lemon juice.
Cover, reduce heat, and cook until tender, about 10 minutes and remove Brussels sprouts and set aside.
In the same pot, melt vegan butter over medium heat, whisk in flour, and cook for 1-2 minutes.
Gradually add vegetable broth, non-dairy milk, salt, and pepper, whisking until smooth.
Bring to a boil, add spaghetti (broken in half), and stir.
Cover and simmer for 8-10 minutes, stirring occasionally, until the pasta is cooked and the sauce thickens.
Add remaining lemon juice, adjust seasoning, and mix in Brussels sprouts and garlic.
Heat for 1-2 minutes more till it thickens and serve with extra lemon juice