To prepare the oranges: Using a sharp knife (I use a chef's knife), cut a thin slice off the top and bottom of each orange so it can stand upright. Working from top to bottom and following the curve of the fruit, use the knife to remove the peel in wide bands, cutting down to the fruit. You want to expose the juicy fruit, so take the thinnest little bit of fruit away with each strip of peel. Carefully run the knife down the connective membranes to release the orange segments one by one. Place the segments between a triple layer of paper towels and let them dry for at least 1 hour, or for several hours, or even overnight. If you have the chance and the towels seem saturated, change them.
To make the Almond Cream: Put the butter and sugar in a food processor and process until the mixture is smooth and satiny. Add the almond flour and process until well blended. Add the all-purpose flour and cornstarch, and process, then add the egg. Process for about 15 seconds more, or until the almond cream is homogenous. Add the rum or vanilla and process just to blend. (If you prefer, you can make the cream in a mixer fitted with the whisk attachment or in a bowl with a rubber spatula. In either case, add the ingredients in the same order.) You can use the almond cream immediately or scrape it into a container and refrigerate it until firm, about 2 hours. It's better if you can allow the cream to chill, but it's not imperative. (The cream can be refrigerated, tightly covered, for up to 3 days.)
To bake: When you're ready to bake, center a rack in the oven and preheat the oven to 350 degrees F. Line a baking sheet with a silicone baking mat or parchment paper and put the tart shell on it.
Stir the almond cream, then turn it into the crust, smoothing the top. Arrange the orange slices in a decorative pattern over the top. Don't cover every bit of cream — it will bubble and rise as it bakes, and it's nice to leave space for it to come up around the fruit.
Bake the tart for 50 or 60 minutes, or until the cream has risen and turned golden brown. If you slip a knife into the cream, it should come out clean. Transfer the tart to a cooling rack and cool to room temperature.
Right before serving, dust the tart with confectioners' sugar and enjoy!