Whisk together lemon juice, mustard, honey, and 1/2 teaspoon of the salt until combined. While whisking, slowly drizzle in 4 tablespoons of the oil to emulsify. Set aside.

Preheat oven to 425°F. Toss together sweet potatoes and 1 tablespoon of the oil, 1/2 teaspoon of the rosemary, and 1/4 teaspoon each of the salt and pepper. Spread in a single layer on one end of a large rimmed baking sheet.

Rub chicken with remaining 1 tablespoon oil, and place on other end of baking sheet, skin side up. Sprinkle with remaining 1/2 teaspoon rosemary, 1/4 teaspoon salt, and 1/4 teaspoon pepper.

Roast in preheated oven until sweet potatoes are tender (stirring potatoes once halfway through) and chicken is golden brown and a thermometer inserted in thickest portion registers 165°F, about 20 minutes. Let sweet potatoes and chicken cool.

Meanwhile, melt butter in a small skillet over medium. Add sugar, and cook, stirring constantly, until dissolved, about 1 minute. Add pecans, and cook, stirring, until sugar mixture no longer sticks to the pan and instead coats the pecans. Remove from heat. Spread pecans in a single layer on parchment paper; cool completely, 10 minutes.

Cut chicken into 1/2-inch slices. Combine kale and sweet potatoes in a large bowl; drizzle with half of the dressing, and toss gently to coat. Divide mixture among 4 plates; top with sliced chicken and candied pecans. Drizzle with remaining dressing.