Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish or two smaller round baking dishes.
Step 2: Prepare the nectarine filling. In a large mixing bowl, combine the diced nectarines, granulated sugar, 1 teaspoon cinnamon, and lemon juice. Sprinkle the cornstarch over the fruit mixture and toss gently until the nectarines are evenly coated. Set aside.
Step 3: Make the streusel topping. In a separate medium bowl, combine the old-fashioned oats, all-purpose flour, brown sugar, 1/2 teaspoon cinnamon, salt, and chopped pecans (if using).
Step 4: Add the cold, diced butter to the dry streusel ingredients. Use your fingertips or a pastry blender to cut the butter into the mixture until it resembles coarse crumbs with some pea-sized clumps.
Step 5: Transfer the prepared nectarine filling into your greased baking dish, spreading it out evenly.
Step 6: Evenly sprinkle the streusel topping over the nectarine filling, ensuring good coverage.
Step 7: Bake for 30-45 minutes, or until the streusel is golden brown and the fruit filling is bubbling thickly around the edges. The fruit should be tender when pierced with a knife.
Step 8: Remove the crisp from the oven and let it cool for at least 20 minutes before serving. This allows the fruit juices to set and prevents the crisp from being too runny.