Preheat the oven to 400°F (200°C).
Peel the garlic and the onion. Cut the onion into slices and place everything on a sheet pan or in a baking dish.
Wash the bell peppers. Cut them into quarters and remove the seeds. Spread them out on the sheet pan (on top of the onion and garlic to prevent them from burning). Drizzle with olive oil and balsamic vinegar. Sprinkle with sea salt and sugar. Roast for 25-30 minutes until soft (see post above for step-by-step pictures).
In the meantime, bring water to a boil for the pasta. Cook them according to the instructions on the package.
Once the bell peppers are roasted, add them to the bowl of a blender. If possible, unscrew part of the lid to let the steam escape while blending (see notes). Otherwise use a stick blender. Blend until smooth.
Drain the pasta and toss them in the sauce. Top with black pepper and sliced almonds if desired.