1. In a 3-qt. saucepan cook potatoes and celeriac in enough lightly salted boiling water to cover about 20 minutes or until tender; drain. Return to saucepan. Cook over high heat 1 minute while shaking pan to dry out potatoes. Add 2 Tbsp. of the butter. Mash potatoes and celeriac until light and fluffy.* Transfer to a large bowl and let cool 20 minutes or until mixture has reached room temperature.

2. Meanwhile, place oven rack in the bottom third of the oven and preheat to 425°F. Butter and flour a 3-qt. rectangular baking dish.

3. In a 10-inch skillet melt the remaining 1 Tbsp. of the butter over medium-low heat. Add onion. Cook, covered, 13 to 15 minutes or until onion is tender, stirring occasionally. Uncover; cook and stir over medium-high heat 3 to 5 minutes more or until golden.

4. Rinse and dry skillet. Add soppressata. Cook over medium heat 3 minutes or until browned and crisp. Transfer to paper towels.

5. Stir caramelized onions, half of the soppressata, and the next nine ingredients (through pepper) into potato mixture. Stir in egg yolks.

6. In a large bowl beat egg whites and cream of tartar with a mixer on medium until stiff peaks form (tips stand straight). Stir one-third of the egg whites into potato mixture to lighten. Fold in the remaining whites. Transfer to the prepared dish.

7. Place dish in oven and reduce temperature to 375°F. Bake 40 to 45 minutes or until golden brown and center is puffed. Serve immediately. Top with the remaining soppressata. Makes 10 servings (1 cup each).

*NOTE If you have one, use a ricer to mash the potatoes and celeriac for fluffier results.

TO BAKE IN A SOUFFLÉ DISH Prepare as directed, except use a 2½- to 3-qt. soufflé or round baking dish with straight sides. Bake 60 to 65 minutes or until puffed and golden brown and the mixture appears set when gently shaken.

 

PER SERVING 252 cal., 16 g fat (9 g sat. fat), 137 mg chol., 477 mg sodium, 14 g carb., 1 g fiber, 4 g sugars, 13 g pro.