Preparation

Roast the rhubarb

• Position a rack in the center of the oven and heat the oven to 400°F.
• Toss the rhubarb with the granulated sugar and salt in a medium bowl. Let sit, stirring occasionally, until the sugar dissolves, about 30 minutes.
• Scrape the rhubarb and any liquid into a 9×13-inch baking dish and arrange in a single layer. Cover the dish with foil and roast for 10 minutes. Turn the pieces over, and roast, covered, until the rhubarb is tender but not falling apart and the syrup is bubbling, about 15 minutes more.
• Let cool slightly, then use a slotted spoon to transfer the rhubarb to a plate, leaving the syrup behind in the dish. Measure the syrup. If it’s more than 1/4 cup, transfer it to a small pot and boil until reduced to about 1/4 cup.

Make the streusel

• Pulse the oats, flour, both sugars, ground ginger, and salt in a food processor until the oats are coarsely chopped. Transfer to a bowl and drizzle with the butter. Using a fork, combine until crumbly with irregularly sized chunks about the size of chickpeas. Mix in the crystallized ginger.

Make the crust; assemble the tart

• Generously butter a 9- to 9-1/2-inch fluted tart pan with a removable bottom.
• Using a fork, combine the flour, brown sugar, ginger, and salt in a medium bowl. Add the butter and stir until combined and large clumps begin to form. Press the dough evenly into the bottom and up the sides of the pan. Freeze for at least 30 minutes.
• Position a rack in the center of the oven and heat the oven to 300°F. Put the frozen crust on a baking sheet and bake until golden brown, 30 to 40 minutes. Remove from the oven and turn the heat up to 350°F.
• Line the warm crust with the roasted rhubarb, snuggling the pieces together in one or two layers. Drizzle the syrup over the rhubarb and top with the streusel, distributing it evenly. Bake until the top is golden brown and the syrup is bubbling, 35 to 40 minutes. Let cool on a rack. Serve warm or at room temperature.

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The rhubarb can be roasted up to 3 days ahead. Refrigerate the rhubarb and the syrup separately. If the syrup becomes too thick to pour, microwave it 1 to 2 minutes on medium before using.

The streusel can be made 2 days ahead; cover and refrigerate.

The shaped frozen crust can be wrapped in plastic wrap and returned to the freezer for up to 1 month before baking.