In a large skillet, drizzle peanut oil over medium heat. Add the onion and cook until softened. Stir in the garlic, then the tomato. Cook until the tomato begins to soften and add the spinach. Cook, stirring often, until wilted.
Mix in the peanut butter and cook until it melts into a sauce. Season with salt, pepper, and cayenne. Serve immediately with rice.