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1
Melt butter in large saucepan over medium heat. Add onions, thyme sprigs, and 1/2 teaspoon salt, stirring to mix. Cover and reduce heat to low. Cook, stirring occasionally, until dark golden brown, about 45 minutes. Remove thyme stems.
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2
Meanwhile, preheat oven to 450u00b0 F. Toss butternut squash with olive oil and 1/2 teaspoon salt and spread out on baking sheet. Bake until browned and very soft, stirring once, about 40 minutes.
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3
Reduce oven heat to 350u00b0 F. Roll dough out until thin and press into a well-greased 10-inch tart pan. Prick all over with a fork and bake in oven 10 minutes.
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4
In a medium mixing bowl, combine eggs, creme fraiche, gruyere, Parmesan, and black pepper.
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5
Spread caramelized onions in bottom of tart and top with butternut squash. Pour gruyere mixture over the squash, making sure to spread it evenly throughout the tart.
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6
Bake until golden brown and cooked through, about 30 to 40 minutes.
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7
Cut butter into 1-tablespoon pieces and freeze for 10 minutes.
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8
Mix flour and salt in food processor or large bowl.
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9
If using food processor, add butter and mix until mixture resembles small pea-sized balls. If mixing by hand, work quickly, mixing the butter in with your hands until it forms small pea-sized balls.
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10
Add in ice water and mix just until dough forms and comes together. Form into disk, wrap in plastic, and place in fridge for 20 minutes.
