Cut bread in half lengthwise; spread cut sides with half of the butter. Set aside.
In a large skillet, cook onions in remaining butter over medium heat for 4-5 minutes or until tender. Add the spinach, garlic powder and hot pepper sauce; cook and stir 3 minutes longer or until spinach is tender.
Spread over bottom half of loaf; sprinkle with cheeses. Replace bread top. Wrap in foil; place on a baking sheet. Bake at 375° for 20 minutes. Open foil; bake 5 minutes longer or until golden brown. Cut loaf into pieces to serve.
TEST KITCHEN TIP French bread rolls can be used in place of the loaf of bread.
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