Combine brown rice with 2 cups water and bring to a boil. Reduce heat to low, cover and let simmer until brown rice is tender and water is absorbed (40-50 minutes).
Meanwhile, preheat oven to 400 degrees F. Wash and dice cauliflower, asparagus and fennel and lay evenly on parchment lined baking sheet. Roast for 20-30 minutes, until fork tender.
Defrost frozen corn and let both the roasted vegetables and brown rice cool to room temperature or chill in the refrigerator.
When ready to assemble, toss the cooked and cooled rice, roasted vegetables and corn together. Mix tahini, lemon, water and seasonings and pour over rice mixture. Toss to combine. Adjust seasonings to taste.