Preheat oven to 400°F.
Cut bell pepper in half lengthwise and remove stem, ribs, and seeds. Cut both halves into ½” wide slices. Then cut the slices in half across their width. Put bell pepper, broccoli, and sesame oil in a mixing well and toss until evenly coated. Add ¼ teaspoon salt and crushed red pepper to the bowl and toss.
Season each chicken thigh with remaining salt, black pepper, and garlic on both sides.
Arrange the chicken and vegetable mixture on a lined baking sheet in one closely packed layer. Bake on center rack for 30 minutes, or until chicken skin is crispy and golden brown.
For Cashew Sauce
Prepare the sauce by adding all ingredients except arrowroot powder to a small saucepan and bring to a simmer, whisking occasionally. Once the sauce reduces, add arrowroot powder for thickness, whisking as you add. Allow to thicken, then remove from heat. Season with salt or pepper as needed.
Garnish dish with scallions, cashews, and basil and drizzle with sauce before serving.