Sweet Potato Casserole with Pecan Streusel

Method

1. 1Roast the sweet potatoes. Heat the oven to 200°C and line a tray with parchment. Prick the potatoes several times and roast them for 55 minutes, until their skins slump and a thin knife slides through the thickest part without resistance. Roasting drives off water and concentrates their flavor, which keeps the finished casserole from turning loose.
2. 2Scoop and mash the flesh. Let the potatoes stand for 10 minutes so they are safe to handle, then split them and scoop 1400 g of hot flesh into a large bowl. Mash until mostly smooth, leaving no firm pieces larger than a pea. Do not use a food processor because it can work the starch into a gluey paste.
3. 3Season the filling. Melt 60 g of the butter and stir it into the mash with the milk, 70 g brown sugar, vanilla, cinnamon, nutmeg and 1 teaspoon salt. Let the mixture cool for 5 minutes, then beat in the eggs one at a time until the filling looks glossy and uniform. Cooling first prevents the eggs from forming cooked flecks.
4. 4Fill the baking dish. Butter a 23×33 cm baking dish lightly with some of the remaining butter. Spread the sweet potato mixture into an even layer and run a spatula over the top without packing it down. An even depth helps the center set at the same time as the edges.
5. 5Make the pecan streusel. Chop the pecans into pieces about 5 mm wide. Rub them together with the flour, the remaining 70 g brown sugar, 0.5 teaspoon salt and 60 g cold butter until irregular crumbs form, with some pieces the size of peas. Scatter the mixture evenly over the filling and leave it loose so hot air can crisp the edges.
6. 6Bake the casserole. Return the dish to the 200°C oven and bake for 25 minutes. It is ready when the filling is hot to the center, the edges rise slightly and the streusel smells toasted and has turned deep golden brown. If the topping is still pale, place it under the grill for exactly 2 minutes and watch it continuously.
7. 7Rest and serve. Leave the casserole on a rack for 15 minutes before spooning it out. The filling will settle into a creamy scoop while the topping stays crisp; serving it straight from the oven makes the center seem thinner than it is. Serve warm alongside roast turkey or ham and a sharp green vegetable.