Put the sunflower seeds in a bowl and cover them with water. Let soak for at least 2 hours, or up to overnight.
Boil a large pot of salted water for the pasta. Once boiling, cook the pasta according to the directions. Drain, return to pot and set aside.
Meanwhile, preheat a saucepan over medium heat and add the oil. Add the carrots, onions, and a pinch of salt. Sauté for about 10 minutes until the onions are translucent and the carrots soften. Add the garlic,c and sauté for another 30 seconds or so.
Place the mixture in a blender. Put the broth, cornstarch, nutritional yeast, tomato paste, ½ teaspoon of salt, and sunflower seeds. Blend until very smooth. This may take up to 5 minutes, depending on how powerful your blender is. Give your blender a break every minute and check how smooth it is. You want it as smooth as possible.
Transfer the sauce back to the saucepan on medium-low heat, and simmer for about 15 minutes until the sauce has thickened. Include the lemon juice and taste for seasoning. Season with more salt if necessary and some freshly ground pepper.
Pour the sauce over the pasta, keeping some to serve over the individual potions. Serve in a bowl and sprinkle with paprika.