Heat the oven to 350°. With cooking spray, coat an 11-inch round tart pan (use one with a removable bottom). To make the crust, combine the flour, walnuts, sugar, and salt in a food processor and pulse until the nuts are finely ground. Add the butter and pulse to incorporate it, then add the cream cheese and blend until the mixture forms a moist ball.
Turn the dough out into the prepared pan. Spread it evenly across the bottom and up the sides of the pan. Bake the crust until it's set but not browned, about 15 minutes. Let it cool on a wire rack.
Meanwhile, make the streusel topping. In a medium bowl, stir together the oats, flour, brown sugar, and walnuts. Cut in the butter until the mixture resembles coarse crumbs.
To make the filling, combine the pumpkin, condensed milk, and cinnamon in a large bowl and whisk until smooth. Whisk in the eggs. Place the tart pan on a baking sheet and pour in the filling (the crust will be very full).
Bake the tart until the filling is just slightly set, about 20 minutes. Remove it from the oven and scatter on the streusel topping. Return the tart to the oven and bake it until the filling is set and the topping is lightly golden, about 25 minutes more. Transfer it to a wire rack and let it cool completely. Remove the tart from the pan before slicing. The tart can be served at room temperature or chilled.