1. To toast the pecans, preheat the oven to 350°F (176°C). Line a cookie sheet with parchment paper or a silicone baking mat.
2. Combine the melted butter, pecans and brown sugar in a medium sized bowl and toss to coat.
3. Spread the pecans evenly onto the cookie sheet and bake for 5-7 minutes, or until toasted.
4. Set pecans aside to cool.
5. Change the oven temperature to 325°F (163°C). Grease a 9×13 pan.
6. Combine the crust ingredients in a small bowl. Press the mixture into the bottom of the prepared pan.
7. Bake the crust for 10 minutes, then set aside to cool.
8. Reduce the oven temperature to 300°F (148°C).
9. In a large bowl, blend the cream cheese, sugar and flour on low speed until well completely combined and smooth. Be sure to use low speed to reduce the amount of air added to the batter, which can cause cracks. Scrape down the sides of the bowl.
10. Add the pumpkin puree, vanilla extract and spices and mix on low speed until well combined.
11. Add eggs one at a time, mixing slowly to combine. Scrape down the sides of the bowl as needed to make sure everything is well combined.
12. Pour the batter into the pan with the crust and spread evenly.
13. Bake the cheesecake for 35-40 minutes.
14. While the cheesecake is baking, make the streusel. Combine all the ingredients for the streusel in a medium bowl, then add the pecans. Mix until crumbly and well combined.
15. Turn off the oven and leave the cheesecake in the oven with the door closed for 10 minutes.
16. Remove the cheesecake from the oven (by this point, the top of the cheesecake should be more firm so it can hold the topping), and sprinkle the pecan streusel over the top.
17. Place the cheesecake back in the oven with the oven door closed for another 15 minutes.
18. Crack the door of the oven, with the cheesecake inside, for 15 minutes. This slow cooling process helps prevent the cheesecake from cracking.
19. Put the cheesecake in the fridge to cool completely, 3-4 hours.
20. Cut the cheesecake into slices and serve.