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1
Place whites and yolks in separate bowls. Let whites warm to room temperature for 1 hour.
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2
Grease bottom of a 15 x 10-1/2 x 1-inch jelly-roll pan; line bottom with waxed paper; then grease with butter. Heat oven to 350 degrees.
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3
Melt 1/3 cup butter in saucepan. Remove from heat. With wire whisk, stir in flour, cayenne, 1/2 teaspoon salt until smooth. Gradually stir in milk. Return to heat, bring to a boil, stirring. Reduce heat; simmer, stirring, until thick and leaves bottom of pan. Beat in 1/2 cup Parmesan and 1/2 cup Cheddar.
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4
With whisk, beat yolks; beat in cheese mixture. With mixer at high speed, beat whites with 1/4 teaspoon cream of tartar until stiff peaks form when beater is slowly raised. With under-and-over motion, fold 1/3 whites into the cheese mixture. Carefully fold in remaining whites to combine. Turn into prepared pan.
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5
Bake 15 minutes, or until surface is puffed and firm when pressed with fingertip.
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6
Meanwhile, make filling - Cook spinach as package label directs. Turn into sieve; press to remove water.
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7
In hot butter in medium skillet, saute onion until golden. Add spinach, salt, 1/4 cup Cheddar and the sour cream; mix well.
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8
With metal spatula, loosen edges of souffle. Invert on waxed paper sprinkled lightly with Parmesan. Peel off waxed paper. Spread surface evenly with filling. From long side, roll up; place, seam side down, on greased cookie sheet. Arrange cheese slices over top. Broil, about 4-inches from heat, just until cheese melts. Use large spatula to remove to serving dish or board.
