The trick to this scrumptious recipe is to be patient with the onions. Cook them slowly on a low heat to let their natural sugars emerge. It’s a perfect tart for brunch.
In a large bowl, mix flour and salt. Cut in shortening and butter with a knife or pastry blender. Sprinkle in 2 to 3 tablespoons cold water. Mix with a fork to moisten. When dough holds together to form a ball, press into a disk, wrap in plastic wrap, and chill for about 30 minutes. Meanwhile, preheat oven to 375°.
On a floured surface, roll out dough so that it’s 2 inches longer and wider than your tart pan. Transfer to pan and tuck and trim edges. Line pastry with foil and fill with dried beans to keep crust from shrinking. Bake 10 to 12 minutes. Remove foil and weights and continue baking until crust is lightly browned, about 10 minutes. Cool to room temperature, then reduce oven heat to 350°.
In a large saute pan over medium heat, cook bacon until done but not crisp–about 8 to 10 minutes. Remove the bacon, reserving grease, and drain on paper towels. When cool, chop into small pieces. Meanwhile, reduce heat to low. Add onions and butter to the pan with the reserved grease. Cook, stirring occasionally, until onions turn a caramel brown color, about 30 minutes. Turn off heat and drain excess fat. Add sage and chopped bacon. Cool to room temperature. Meanwhile, beat eggs and half-and-half together; add salt and pepper. Set aside.
Spoon onion and bacon mixture into pastry shell; pour egg and cream mixture over it. Bake until tart is set, about 45 minutes to an hour. Cool at least 5 minutes before serving. Garnish as desired.