In a large wide skillet over medium heat, melt the unsalted butter with the neutral oil until the butter foams, 1–2 minutes.
Add the onion and cook, stirring, until translucent and lightly golden, 5–7 minutes.
Stir in the garlic and cook until fragrant, 30–60 seconds.
Sprinkle in the cassava flour (farinha de mandioca) in a steady rain, stirring constantly to coat; toast over medium to medium-low heat until the grains are sandy, loose, and lightly golden, 5–10 minutes.
Season with the kosher salt and black pepper; taste and adjust if needed.
Off the heat, fold in the fresh parsley. Serve warm alongside beans, grilled meats, or stews.