Combine the first seven ingredients in a small bowl to make the Seasoning Mix.
Oil both sides of the fish fillets with 2 teaspoons olive oil. Season the fish fillets with ½ teaspoon of the Seasoning Mix per side. Quickly brown the fillets on both sides, but do not cook through. Then remove and set the fillets aside.
In a heavy 8-inch nonstick skillet over high heat combine the onions, bell peppers, celery, and 1 tablespoon of the olive oil. Cook, stirring frequently, until the vegetables just begin to brown and stick to the pot, about 8 minutes. Reduce the heat to medium-low and add the remaining 1 teaspoon olive oil-cooking the vegetables slowly like this gives them a sweet taste that is perfect for this dish. Stir well, then add 2 teaspoons Seasoning Mix. Continue to cook, stirring frequently, for 5 minutes, then remove from the heat. The taste begins with a mild flavor of the cooked vegetables, followed by a combination of gentle sweetness and a hint of white pepper from the Magic Seasoning. The vegetables are still slightly crunchy at this point. Set the vegetables aside.
In a 4-quart saucepan, combine 1 tablespoon plus 1 teaspoon of the butter and the cream over high heat and bring to a boil. Add 3 cups of the stock, the potatoes, garlic, and the remaining Seasoning Mix. Cover and return to a boil, then remove the cover and cook until the potatoes are tender, about 8 minutes.
Remove ½ cup of the liquid and set it aside. Mash the potato mixture, then add ½ cup stock and the reserved vegetables (don't wash the skillet in which the vegetables were cooked-you'll use it for the sauce), and stir until well blended. Remove and reserve ¾ cup of this mixture and set aside to cool.
Preheat broiler to 500ºF.
Spray a sheet pan lightly with the vegetable oil, and arrange the tilapia fillets on it. Mound 1 cup of the stuffing mixture on top of each fillet, and place in broiler. Broil until the fish is white and opaque all the way through and the stuffing is nicely browned around the edges, about 8-10 minutes, or until fish is cooked through. Keep fillets warm while you make the sauce.
Deglaze the 8-inch skillet with ½ cup of the stock, then add the reserved liquid from the potatoes, the reserved potato mixture, the yogurt and the remaining butter. Bring just to a boil, reduce the heat to low, then simmer for 5 minutes and remove from the heat. Makes about 1½ cups. Pour the sauce over the fillets and serve immediately.
NOTE: Elephant garlic, which we specify, is much larger than regular garlic, and also milder. If you use regular garlic, you might want to reduce the amount.