In a medium saucepan, bring the milk to a boil. Stir in the oats and remove from the heat. In a small bowl, combine the sour cream, horseradish, and 1/4 teaspoon of the salt.
In a large nonstick frying pan melt 1 tablespoon of the butter with 1 tablespoon of the oil over moderate heat. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Add the carrot and cashews and cook, stirring occasionally, until the carrot is tender, about 5 minutes longer. Remove from the heat and stir in the oats.
In a bowl, mix the parsley, eggs, 1 teaspoon of salt, 1/2 teaspoon of pepper, and the oat mixture. Heat 1 tablespoon of the oil in the frying pan over moderate heat. Using a 1/4-cup measure, scoop mounds of the oat mixture into the pan and flatten with a spatula. Fry in batches, adding the remaining oil as needed, until golden, about 3 minutes per side. Keep warm in a 200°F oven on a baking sheet lined with paper towels.
Meanwhile, in a medium saucepan, melt the remaining tablespoon of butter over moderately low heat. Add the spinach and the remaining 1/2 teaspoon each of salt and pepper. Cover and cook until hot, about 5 minutes. Serve with the oat cakes with the sauce on the side.