Prepare the pastry: In a large bowl, whisk together flour and 3 tbsp sugar. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs, with some pea-sized pieces remaining.

Gradually add ice water, one tablespoon at a time, mixing until the dough just comes together. Form into a disc, wrap in plastic, and chill in the refrigerator for at least 30 minutes.

Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough to fit a 9-inch tart pan with a removable bottom. Press the dough into the pan, trim edges, and prick the bottom lightly with a fork.

Prepare the filling: In a bowl, gently toss the sliced nectarines with 1/4 cup sugar, lemon juice, and cornstarch until evenly coated. Arrange the fruit neatly in the prepared crust.

Make the crumb topping: In a separate small bowl, combine flour, oats, brown sugar, and cinnamon. Cut in the 4 tbsp cold butter until the mixture forms moist crumbles. Sprinkle the topping evenly over the nectarines.

Bake for 45-50 minutes, or until the crust is golden brown and the filling is bubbling. If the topping browns too quickly, loosely cover with foil.

Let the tart cool on a wire rack for at least 20 minutes before carefully removing the outer ring. Serve warm or at room temperature.