Preheat oven to 400°F (200°C).
Place red bell peppers cut-side down on a foil lined baking sheet.
Bake for 20-30 minutes, or until the skin blackens.
In the meantime, prepare the pasta according to package instructions.
While water heats, prep the rest of the sauce. Combine avocado, almond milk, lemon juice, garlic, salt, pepper, red pepper flakes, olive oil (optional), and nutritional yeast in a blender.
While pasta cooks, saute the chopped onion in a pan, set aside.
Pull the peppers from the oven. Transfer the peppers into a lidded container. Let cool covered for 5 min. Once cool enough to handle, peel the blackened skins from the peppers and place into a blender or food processor. Discard the skins.
Add the roasted red peppers to the blender. Blend until smooth.
Combine with cooked pasta and sauteed onion. Serve.