Combine reserved liquid from mandarin oranges, canola oil, olive oil, mustard, cloves, and Chinese five-spice powder in a jar with a tight-fitting lid; seal jar and shake vigorously until the dressing emulsifies.
Melt butter in a saucepan over medium heat. Stir pecans and sugar into the butter; let simmer until hot, 1 to 2 minutes. Season the pecans with rosemary and cayenne pepper; continue cooking another 1 to 2 minutes. Remove from heat and drain pecans on a plate lined with paper towels.
Toss spinach leaves and green onion together in a large mixing bowl; top with mandarin oranges and pecans. Drizzle dressing over the salad to serve.