To start, preheat your oven to 450°F (232°C). Then, lightly butter your baking dish and set it aside.
Next, measure out all of the topping ingredients. Combine everything except for the butter and mix well. Then, cut the butter into 1/2in (1.3cm) cubes. Add the butter pieces into the bowl of topping mix and combine with your hands by smushing and rubbing the butter into the dry ingredients. Stop when no dry ingredients remain unincorporated and the blend forms clumps between bean and pea-sized pieces. Don't overmix. If the topping is too uniform, the mix won't dry out correctly while baking. Place the topping in the refrigerator to chill while you make the filling.
For the filling, whisk together the following filling ingredients into a large bowl: brown sugar; granulated sugar; flour; cinnamon; salt; and zest. Set the bowl aside. Next, De-pit and slice your nectarines into ½-1in (1.3-2.5cm) thick wedges (see FAQs for help). Then, add them to your dry filling mix and gently toss to coat with a large spoon. In a small bowl, make a cornstarch slurry by whisking the lemon juice and cornstarch together. Pour the slurry over the coated nectarines and gently toss once more.
To assemble, add the filling to your buttered baking dish. Then, take the topping out of the fridge and cover the nectarines with half of the mixture. Set aside the remaining topping.
Now, place the dish on a baking sheet (in case of spillage) and put the crisp into the middle rack of your oven. Immediately reduce the oven temperature to 350°F (177°C). Bake the crisp for 30 minutes. Then, take it out, add the rest of the crisp topping, and bake for another 30 minutes. Let the crisp cool and set for 15 minutes before serving.