Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Line a large roaster with parchment paper.

Lay duck in roaster spread out flat, skin side up.  Sprinkle generously with salt and pepper, rubbing it in.

Bake for 30–45 minutes, uncovered, until the skin looks crispy.

Meanwhile, sauté the onions and celery in olive oil over medium heat for 20 minutes, stirring occasionally.

Add brown sugar towards the end, and thyme right before removing from heat.

Place prunes in a small bowl. Add lemon juice, honey, and orange zest and mix well. Let it sit for a while to absorb flavors. (This can be done the day before and refrigerated until using.)

Remove duck from oven. Mix onions and prunes together and pour all over duck. Pour wine over all. Add the sherry and bay leaf.

Cover the roaster with parchment paper and seal as hermetically as possible with two layers of aluminum foil.

Return to oven for two and a half hours. Let cool slightly before cutting into portions.