Preheat the oven to 425°F (220°C). Prep a large baking sheet with parchment or a light coating of oil.
In a small bowl, mix together cumin, paprika, coriander, cinnamon, turmeric, red pepper flakes (if using), salt, and pepper. Stir to combine.
Pat the chicken thighs dry with a towel. Rub the spice mix all over the thighs, under the skin when possible, so every bite feels seasoned.
Toss the sweet potato pieces with a tablespoon of olive oil and a pinch of salt. Spread them in an even layer on the baking sheet.
Place the seasoned chicken thighs, skin-side up, on the baking sheet among the sweet potatoes.
Drizzle the remaining olive oil over the chicken and potatoes. Scatter minced garlic over everything.
Pour water or chicken broth into the sheet pan to create a little steam for tender sweet potatoes.
Roast for 25 minutes, then baste the chicken with the pan juices. Flip the potatoes if needed for even browning.
Return to the oven and roast for another 15–20 minutes, or until the chicken reaches 165°F (74°C) and the potatoes are tender.
If you like a bit more color, you can broil for 1–2 minutes at the end, watching closely so nothing burns.
Remove from the oven and squeeze lemon juice over the dish. Garnish with chopped parsley if you have it. Serve warm.