Optional for serving: vanilla ice cream or frozen yogurt, whipped cream, or Greek yogurt
Coat a 5- to 6-quart slow cooker with nonstick spray or line with a nonstick liner. In a large bowl, combine the oats, whole wheat flour, almonds (if using), cinnamon, and salt. Pour the melted butter and maple syrup over the top, then stir to evenly moisten.
If using frozen berries, rinse them so that any large ice crystals are removed (the berries do not need to be completely thawed). Place in a large bowl, then stir together with the lemon juice, vanilla, and 1/2 tablespoon cornstarch (if using fresh berries) or 1 tablespoon cornstarch (if using frozen). Transfer to the prepared slow cooker, then sprinkle the oat mixture evenly over the top. Lightly coat the top of the crisp with nonstick spray.
Lay a sheet of paper towel or kitchen towel over the top of the slow cooker so that it overhangs the sides and will stay in place once the slow cooker is covered**. (This absorbs moisture and helps the top to crisp). Cover and cook for 1 to 2 hours on high or 2 to 3 hours on low, until the fruit is bubbly and the top is browned. If your berries are still frozen, the cooking time may be longer. Serve warm, topped with vanilla ice cream, Greek yogurt, or whipped cream as desired.
**DISCLAIMER: Please note that using a towel with a slow cooker could be a fire hazard. Proceed at your own risk. I have done it many times over the years with different recipes and different slow cookers and have never had an issue. That said, different slow cookers can reach different temperatures. If this is your first time using the towel method with your slow cooker, I recommend checking the towel an hour into cooking to be sure that it isn’t too hot.