Preheat oven to 400°F (200°C) and line a large baking sheet with parchment paper.
In a large bowl, toss the cubed chicken and sweet potatoes with olive oil, garlic powder, smoked paprika, salt, and pepper until evenly coated.
Spread the chicken and sweet potatoes in a single layer on the prepared baking sheet.
Roast for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the sweet potatoes are fork-tender.
While roasting, cook the brown rice in 4 cups of water or chicken broth until tender and liquid is absorbed.
Assemble the bowls by placing 1 cup of fresh spinach at the bottom of each of the four containers.
Add 1/2 cup of cooked brown rice to each bowl.
Divide the roasted chicken and sweet potato mixture evenly among the bowls.
Top with fresh avocado slices and drizzle with lemon tahini dressing before serving.