Add a small amount of olive oil to a pot and heat over medium heat.

Add the sliced onion to the pot and sauté until lightly browned.

Carefully add the gefilte fish pieces and the gelatin from the jar to the pot.

Add sliced carrots, chopped celery, and black pepper to taste to the pot.

Increase the heat to high and bring the mixture to a boil, then reduce the heat to medium-low.

Simmer the mixture for 8 to 10 minutes. Remove the pot from the heat.

Gently remove the fish pieces from the pot and place them on a platter to cool.

Reserve the vegetables from the pot.

Lightly oil a cookie sheet.

Place the cooled fish pieces on the oiled cookie sheet, turning to coat both sides with oil.

Preheat oven to 350 degrees Fahrenheit.

Bake the fish for 10 minutes.

Turn the fish over and bake for another 10 minutes, or until lightly browned.

Remove the fish from the oven and refrigerate until chilled.

Serve the chilled gefilte fish with the reserved vegetables and horseradish.