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To make the crust: In a medium-sized bowl, whisk together the flour, sugar, lemon zest or oil, and salt.
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Using a pastry fork or pastry blender, your fingers, or an electric mixer, cut in the cold butter, working the mixture until it's crumbly; a few larger chunks of butter can remain.
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In a small cup or bowl, whisk together the egg yolk and the vanilla. Stir this mixture into the pastry dough.
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Roll or press the dough onto the bottom and at least 1" up the sides of a 9" to 10" springform pan or tart pan (preferably with a removable bottom), or 9" round cake pan.
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Use a fork to prick the dough all over (this will keep it from puffing up as it bakes), and refrigerate it for 30 minutes or longer.
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Bake the chilled crust for 16 to 18 minutes in a preheated 375°F oven, until it's golden brown. Check it midway through the baking time; if it's developed any big bubbles, puncture them.
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Remove the crust from the oven (leave the oven on), and cool it on a rack as you prepare the filling.
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To make the lemon filling: In a medium-sized bowl, or the bowl of an electric mixer, mix together the sour cream and sugar.
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Whisk in the eggs, then add the lemon juice and lemon zest or oil.
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Pour the filling into the cooled tart shell. Don't overfill the shell; if any filling seeps over the top edge, it'll burn and stick.
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Return the tart to the 375°F oven, and bake it for 18 to 20 minutes. The filling should look "set," but still wobble slightly in the middle.
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Remove the tart from the oven, and set it on a rack to cool for 1 hour.
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Refrigerate the tart; top it with berries and glaze a few hours before serving.
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To make the berry glaze (if using): In a saucepan or microwave-safe bowl, combine the brown sugar and juice.
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Sprinkle the gelatin over the juice, and let it swell and soften for 5 minutes or so.
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Heat the mixture over low heat or in the microwave until the gelatin dissolves.
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Remove it from the heat and let it cool for about 30 minutes, until it's lukewarm.
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To assemble: If you're using strawberries, wash, trim the top of the strawberries to remove their leaves, and slice them; you can leave them whole if they're small.
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Brush a thin layer of glaze (about 2 tablespoons) over the tart.
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Mix the berries with the remaining glaze, then arrange them, one by one, atop the tart. You'll have some glaze left in the bottom of the bowl; pour it into a small container and keep it at room temperature.
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Place the tart in the refrigerator for the glaze to set.
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After about 30 minutes of chilling, remove the tart from the fridge, and brush the berries with the reserved glaze. This will keep them fresh and glistening for 12 hours.
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Storage information: Store, covered, in the fridge for 3 days.
