1. Preheat the oven to 400°F. Grease a 3-quart ovenproof baking dish.
2. Peel and quarter the onion. Place in the bowl fitted with the ultrablade knife and mix at speed 12 for 10 seconds. Scrape down the sides of the bowl.
3. Dice the carrot and celery. Replace the ultrablade knife with the mixer. Add the carrot, celery and oil. Launch the P1 simmer program at 270°F for 5 minutes. Scrape down the sides of the bowl.
4. Add ground lamb, breaking into pieces, and launch the P3 simmer program for 10 minutes. Drain the excess liquid from the bowl. Add remaining ingredients, except cornstarch, water and potatoes. Season with salt and pepper. Launch the P3 simmer program at 200°F for 12 minutes. Stir together water and cornstarch. After 10 minutes, pour cornstarch mixture through the opening on the lid and continue to cook remaining 2 minutes.
5. Spoon the mixture into a prepared dish and top with the mashed potatoes. Bake for 15 minutes or until heated through and filling is bubbly.