Preheat oven to 425°F.
Stir together water, sugar, ¾ cup of the vinegar, and 1 teaspoon of the salt in a small saucepan; bring to a boil over high.
Reduce heat to medium-low; simmer, undisturbed, until sugar is dissolved, 2 to 3 minutes.
Remove from heat. Place onion slices in a heatproof jar or bowl; carefully pour pickling mixture over onion, ensuring slices are fully submerged.
Seal jar with lid, or cover bowl with plastic wrap; let stand at room temperature until ready to use.
Toss together sweet potatoes, smoked paprika, cumin, 1 teaspoon of the salt, and 2 tablespoons of the oil in a large bowl until well combined.
Arrange sweet potatoes in an even layer on a baking sheet. Roast in preheated oven until fork-tender, 20 to 25 minutes. Remove from oven, and set aside.
Whisk together hot honey, mustard, pepper, and remaining 2 tablespoons vinegar, ¼ teaspoon salt, and 2 tablespoons oil in a large bowl until smooth and combined.
Drain pickled onion to measure ½ cup drained onion; reserve remaining pickled onion and pickling liquid in a sealed jar in refrigerator up to 1 month.
Add sweet potatoes, parsley, and drained pickled onion to hot honey mixture in bowl; toss until evenly combined.
Sprinkle salad with pecans, and serve.
