Drain cashews and place in a blender with 1/4 cup water. Blend until smooth; set aside.
Heat oil in a medium frying pan over medium heat. Add onion and 1/2 tsp. salt and cook, stirring often, until onion is caramelized, 8 to 10 minutes; set aside.
Meanwhile, bring 3 cups broth to a boil in a medium saucepot. Add grits in a slow but steady stream, whisking constantly, until mixture is smooth. When mixture comes to a boil, reduce heat to low.
Simmer grits, stirring occasionally, until most of the liquid has been absorbed, 5 minutes. Add remaining 1 cup broth and 1/2 tsp. salt and cook, whisking constantly, until thickened, another 10 minutes.
Stir in creamed cashews and onions. Cover and cook, stirring occasionally, until grits are soft and fluffy, about 30 minutes.
Make ahead: Since the cashews take a while to make, it can be worth blending a few batches and then either chilling (up to 4 days) or freezing (up to 3 months) in an airtight container.