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1.
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Caramelizing the onions.
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Peel the onions and cut them in half from root to tip.
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Cut out the dense core at the root end and slice the onions 1/4 inch thick, again from root end to tip.
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Cook the bacon, stirring often, in a large (4-quart) saucepan over medium heat until almost crisp.
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Add the onions, sugar, and salt, and cook, stirring often until they cook down by two-thirds, about 10 minutes.
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Add 2 teaspoons vinegar, reduce the heat to medium-low, and continue to cook until the onions are an even golden brown and softened to a marmalade consistency, 15 to 30 minutes, depending on the onions.
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Stir often and scrape up any brown bits clinging to the bottom of the pan.
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The onions need almost constant stirring near the end to prevent them from sticking and burning.
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They'll let you know they need attention by giving off a sizzling sound.
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Stir in the sage, allow them to cool slightly, then taste and season with black pepper and additional salt if needed.
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If the onions seem overly sweet, stir in another teaspoon of vinegar.
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(The onions can be caramelized up to 2 days ahead and store covered in the refrigerator.)
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2.
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Filling and baking.
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Preheat the oven to 350F.
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Stir the cream and egg into the caramelized onions until thoroughly combined.
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Divide the filling among the tart shells and spread it evenly with the back of a spoon.
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Bake in the upper third of the oven until the filling is set, about 15 minutes.
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The filling should still be soft but not runny.
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Let cool slightly, then transfer the tarts to a cutting board using a large spatula.
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Cut each into 8 wedges with the downward pressure of a sharp chef's knife.
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Serve warm or at room temperature.
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For large tarts, prebake 2 10-inch Free-Form Tart Shells.
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Divide the onion mixture between them and bake the tarts until the filling is set in the center, 20 to 25 minutes.
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Using a large spatula, transfer them to a cutting board and cut each into 12 wedges.
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In place of sage, use an equal amount of finely chopped rosemary, marjoram, savory, English thyme, or lemon thyme.
