Place the cashews in a small saucepan and cover them with water. Bring to a boil and boil 15 minutes. Then drain and rinse them under cold water until cool.
Meanwhile, in a small bowl, mix 1/2 cup oat milk with 2 tablespoons cornstarch and set aside.
Add 1 1/2 cups oat milk to a medium saucepan with the coconut milk. Warm the mixture over medium low heat for 2 minutes, whisking to incorporate the solids. Then add the agave syrup and sugar. Whisk in the cornstarch mixture and continue on medium low until the mixture just starts to bubble and is thickened, about 3 to 5 minutes. Remove from heat and stir in the vanilla extract and salt.
In a blender, combine the oat milk mixture with the cashews and blend until completely smooth and no chunks remain. (A high powered blender is best. If you see visible chunks, strain through a fine mesh strainer before going to the next step.)
Transfer the mixture to a large metal or glass bowl. Place the bowl in a smaller bowl of ice water for 1 hour, making sure not to splash any water into the custard, until the temperature drops to 40°F. Alternatively, simply place the bowl in the refrigerator and wait for 3 hours or overnight.
When the custard is chilled, pour it into an ice cream maker and churn according to the ice cream maker instructions, about 25 minutes for soft serve consistency. For a hard ice cream texture, transfer to a loaf pan or sealed container and freeze in the freezer for at least 2 hours (or up to 4 hours). Store in a loaf pan with a parchment sheet over the top or a sealed container for up to 2 weeks. Important: If frozen for over 4 hours, allow it to stand at room temperature for about 20 minutes before serving to soften.