Preheat the oven to 210 °C (410 °F, top and bottom heat).
Prepare the Brussels sprouts: Wash them, trim the stem slightly, and remove any outer leaves if needed.
Check the size: If the sprouts are small, leave them whole. If they’re larger, halve them. Place them in a bowl.
Prepare the onion and garlic: Peel the red onion and cut into half rings. Peel the garlic and leave whole, just trim the hard ends. Add both to the bowl with the Brussels sprouts.
Make the marinade: Mix about 3 tbsp olive oil, ¾ tsp salt, cumin, chili, black pepper, and agave syrup.
Coat the vegetables: Pour the marinade over the Brussels sprouts, onion, and garlic, and mix with your hands so everything is evenly coated.
Roast the vegetables: Spread the vegetables on a baking tray lined with parchment paper and roast in the oven for about 15 minutes, until the Brussels sprouts start to brown.
Meanwhile, toast the sunflower seeds in a non-stick pan without oil until lightly browned. Let them cool on a plate.
Cook the pasta according to the package instructions.
Make the pesto: Blend half of the roasted Brussels sprouts with all the roasted onion and garlic, the remaining salt, about 6 tbsp olive oil, lemon juice and zest, parsley, yeast flakes, and 6 tbsp of the pasta cooking water. A food processor works best.
Combine pasta and pesto: Mix the cooked pasta with the pesto, adding about 2 tbsp olive oil.
Serve: Top the pasta with the remaining roasted Brussels sprouts.