Preheat the oven to 425ºF.
Cook the quinoa: Transfer the washed quinoa to a saucepan with 2 ½ cups water or vegetable broth. Bring the liquid to a simmer and cook covered for 15 minutes. Set aside for 5 minutes before fluffing with a fork.
Prepare and roast the veggies: Meanwhile, transfer the Brussels sprouts and sweet potatoes to a large rimmed baking sheet in a single layer. Toss them with olive oil, onion and garlic powders, and salt. Place them in the oven on the middle rack. Roast for about 25 minutes, or until the sweet potatoes are just fork tender and golden brown.
Prepare and roast the lentils: Transfer the cooked lentils to a medium baking sheet in a single layer. Toss them with olive oil, onion and garlic powders, and salt. Place them in the oven on the top rack. Roast them for about 15 minutes, or until they are crispy.
Make the dressing: Transfer the tahini, cumin, salt, and lemon juice to a medium bowl. Grate or press the garlic into the bowl. Add the ice-cold water 1 tablespoon at a time until the desired consistency.
For serving: Serve the quinoa, roasted lentils, and veggies with the desired amount of dressing. Season with salt and pepper. Top with chopped avocado and a sprinkle of hemp seeds, if using.