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1
Melt butter in small saucepan over medium heat. Whisk in flour. Cook 30 seconds, whisking constantly. Whisk in milk and nutmeg. Simmer until sauce thickens and comes to boil, 1 to 2 minutes. Season to taste with salt. DO AHEAD:
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2
Preheat oven to 350u00b0F. Heat chicken fat in large ovenproof skillet over medium-high heat. Add onions; sprinkle with salt. Saute until onions are light golden, about 14 minutes. Stir in bay leaves and thyme sprig; dot with butter. Place skillet in oven; bake until onions are dark golden, stirring often, about 30 minutes. Transfer onions to bowl; discard bay leaves and thyme sprig. DO AHEAD:
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3
Preheat oven to 375u00b0F. Line large baking sheet with parchment paper. Soak potato slices in large bowl of cold water 15 minutes. Drain well; pat dry.
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4
Add enough oil to heavy medium skillet to reach depth of 3/4 inch. Place over medium heat; attach deep-fry thermometer and heat oil to 325u00b0F. Working in batches, add potato slices to oil and cook just until softened but not brown, about 2 minutes per batch. Using slotted spoon, transfer potato slices to prepared baking sheet in single layer and sprinkle with salt and pepper.
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5
Brush inside of four 4-inch-diameter tart pans with melted butter. Line each pan with overlapping potato slices. Cover bottom of each potato crust with 1/4 of onions; sprinkle with chopped thyme. Spread 1 generous tablespoon bechamel over. Sprinkle each with 1 tablespoon goat cheese.
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6
Place tarts on small baking sheet. Bake until filling is heated through and potato crust browns, 18 to 20 minutes. Sprinkle each with remaining goat cheese.
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7
Push up pan bottoms, releasing tarts. Arrange tarts on plates. Spoon Heirloom Tomato Salad alongside and serve.
