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Photo, Roberto Caruso.
2 cups all-purpose flour
1/2 tsp salt
3/4 cup cold unsalted butter, cubed
1 tsp white vinegar
1 tbsp vegetable oil
1 red onion, sliced
4 shallots, peeled and sliced
1/2 tsp salt
1/2 tsp granulated sugar
4 eggs
1/2 cup 35% cream
1/4 cup milk
2 cups grated gruyère
1 tsp fresh thyme leaves
WHIRL flour with salt in a food processor. Add butter and pulse until coarse crumbs form. Gradually add 1/4 cup ice water and vinegar, pulsing after each addition. Pulse just until dough starts to come together. Gather into a ball, then press into a disc. Wrap in plastic wrap. Refrigerate at least 45 min or up to 3 days.
HEAT a large non-stick frying pan over medium. Add oil, then onions and shallots. Cook, without stirring, for 5 min. Reduce heat to medium-low. Add salt and sugar. Season with pepper. Cook, stirring often, until onions turn deep brown, 6 to 12 more min. Set aside.
WHISK eggs with cream and milk in a medium bowl until smooth. Set aside.
PREHEAT oven to 400F. Lay an 11-in. fluted tart pan on a baking sheet. Cut out a foot-long piece of parchment paper. Roll dough on a lightly floured surface into a 13-in. circle, about 1/4 in. thick. Carefully lift dough into tart pan. Gently press dough into the corners and up the edges of the pan. Trim and discard excess. Prick the dough with a fork about 15 times. Lay parchment on the shell and fill with 2 cups dry beans or pie weights.
BAKE in centre of oven for 15 min, then cover with foil. Continue baking until shell is just golden, about 15 more min. Remove to a rack. Reduce heat to 350F. Lift out parchment and beans. Reserve beans for future use.
SPRINKLE half of cheese, then onion mixture over warm crust. Add egg mixture. Sprinkle with remaining cheese, then thyme.
BAKE in centre of oven until top is firm, 20 to 22 min. Transfer to a rack.Let tart rest 10 min before serving.
Pre-baking a tart shell or "blind baking" with dry beans or pie weights helps prevent the crust from shrinking or puffing up while baking.
Calories 382, Protein 10g, Carbohydrates 26g, Fat 27g, Fibre 2g, Sodium 431mg.