Sauté Brussels Sprouts & Garlic: Heat olive oil in a large pot over medium-high heat. Add garlic and Brussels sprouts. Sauté for 5 minutes.
Season & Steam: Add 1 tablespoon lemon juice, salt, and pepper. Cover and cook on medium-low for 10 minutes until tender. Remove and set aside.
Make the Sauce: Melt vegan butter in the same pot. Whisk in flour and cook for 1-2 minutes.
Add Liquid & Pasta: Slowly whisk in vegetable broth, non-dairy milk, salt, and pepper. Bring to a boil, then add spaghetti (break in half if needed).
Cook Pasta: Reduce heat to medium, cover, and simmer for 8-10 minutes. Stir occasionally, pressing pasta into the liquid.
Finish: Once the sauce thickens, stir in the remaining lemon juice and adjust seasoning. Return Brussels sprouts and garlic to the pot, mix well, and heat for 1-2 minutes.
Serve: Enjoy with extra lemon juice and vegan Parmesan, if desired.