Place whites and yolks in separate bowls. Let whites warm to room temperature for 1 hour.
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.
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.
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.
Bake 15 minutes, or until surface is puffed and firm when pressed with fingertip.
Meanwhile, make filling - Cook spinach as package label directs. Turn into sieve; press to remove water.
In hot butter in medium skillet, sauté onion until golden. Add spinach, salt, 1/4 cup Cheddar and the sour cream; mix well.
With metal spatula, loosen edges of soufflé. 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.