Preheat the oven to 425°F. Toss the sweet potato, bell pepper, and broccoli with 2 tbsp olive oil, salt, and pepper. Spread on a sheet pan and roast 22-25 minutes, until charred and tender.

While the vegetables roast, cook the quinoa: combine the rinsed quinoa and stock in a small saucepan. Bring to a boil, reduce to a simmer, cover, and cook 15 minutes. Remove from heat; let stand 5 minutes. Fluff with a fork.

Make the lemon-tahini: whisk the tahini with 2 tbsp water until smooth. Add the lemon juice and grated garlic. Thin with more water, 1 tsp at a time, until pourable. Season with salt.

Build the bowls: divide the quinoa among 4 bowls. Top with the roasted vegetables, avocado slices, and pickled red onions.

Drizzle with the lemon-tahini. Scatter the parsley and mint over the top. Serve warm or at room temperature.