Preheat the oven to 350 F. Butter a deep dish 9-inch pie pan or a baking dish large enough to hold the fruit, or spray with nonstick cooking spray
Place the nectarines and plums in the pie pan and sprinkle over the lemon juice and the cinnamon. Toss to combine.
In a medium bowl, combine the flour, brown sugar, and salt. Use your fingers, two knives, or a pastry cutter to blend in the butter so that the mixture has a lightly bumpy consistency. Stir in the oats. Distribute the topping over the fruit. Place a foil or parchment paper-lined rimmed baking sheet on the lower rack of an oven, then place the pie on a rack in the middle of the oven.
Bake for about 40 minutes until the fruit is bubbling and the top is browned. A knife should slide easily into the plums (cover the crisp with foil if the top is browning too much before the fruit is cooked). Cool for 15 to 30 minutes on a wire rack, and serve warm with vanilla ice cream or sweetened whipped cream.
Notes
If the top is browning too quickly, tent it with foil.
If you prefer to use a food processor to make the streusel, that’s an option for sure. Just combine the flour, brown sugar, and salt in the food processor, and pulse in the bits of cold butter until you have a pebbly consistency. Turn the mixture into a bowl, and stir in the oats (so they don’t get all chopped up–yes, this dirties more bowls. You decide).
The crisp will keep covered with plastic wrap in the fridge for up to 5 days. To reheat the crisp, heat it in a preheated 300-degree oven until hot. Tent the top with foil if it's getting too browned. Avoid reheating crisp in the microwave, which will make the top soggy.