In a medium bowl, stir hot water into the Jell-O powdered gelatin until it is completely dissolved. Add the mandarin orange juice and the softened sherbet and continue to mix until the sherbet is melted.
Spray a jello mold (or an 8x8-inch glass pan) with nonstick spray. Pour the Jell-O mixture into the mold and gently scatter the orange slices into the mixture.
Refrigerate for several hours (or overnight) until well set. Carefully turn out the chilled jello mold onto a decorative plate (see tips below). Or if you used a square dish, you can just put it out for scooping and serving. Keep refrigerated until ready to serve.
Notes
*Tips for getting the Jell-O out of the mold: Use a small rubber spatula or a knife to loosen the edges of the salad. Place a serving plate on top of the mold and gently flip it over. If the Jell-O doesn't seem to be coming out, you can always place the bottom of the pan in warm water for about 10 seconds, then flip it over. It should slide right out.
Softened sherbet mixes in more evenly — let it sit at room temperature for 5 to 10 minutes before adding it in.
Make-Ahead Tip: this Jello salad is an ideal make-ahead dessert! Mix it, chill it, and forget it. It’s best served cold, so overnight chilling makes it even better. If you’re using a mold, let it set for at least 6 hours (or overnight) for perfect results.
For a fun presentation, garnish with whipped cream, fresh mint, or extra mandarin orange slices.
You can substitute other flavors of sherbet (like lime or raspberry) for a colorful twist.