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Heat butter over medium heat in a large skillet until hot. Add onions, reduce heat and cook over low heat stirring frequently, until onions are soft and lightly browned. This will take approximately 40 minutes. Season to taste with salt and pepper. Remove skillet from the heat, cool, cover and refrigerate. Onions can be prepared up to 3 days in advanced and kept covered in the refrigerator until ready to use.
Place heavy cream, thyme, orange zest and pepper in a medium size pan and bring to a boil. Remove pan from heat, cool for 15 minutes. Remove the thyme sprigs.
Cut sweet potatoes into thin slices. Butter a 13x9 in (13x9 cm) baking dish and arrange 2 layers of sweet potato slices, slightly overlapping, on the bottom of the pan. Season with salt and pepper. Spread 1/3 of the onions on top of the sweet potatoes and drizzle 1/3 of the heavy cream preparation. Repeat preparation 2 more times, ending with cream preparation. Press down layers with your hand. Cover pan tightly with aluminum foil.
Bake in preheated oven at 350ºF (180ºC) for 1 hour. Remove foil and continue cooking until sweet potatoes are soft and tender.
Process or finely chop pecans, breadcrumbs and parmesan cheese. Cover potatoes with the pecan preparation, increase oven to 375ºF (190ºC) and return pan to the oven to broil for 10 minutes or until top is golden.
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