Reduce the heat to medium and add the butter. When the butter has melted, add the onion and garlic, season with salt and pepper, and cook, stirring occasionally, until softened, about 4 to 5 minutes. Add the beef, season with salt and pepper. Cook beef stirring occasionally, until it’s just cooked through and no longer pink, about 6 to 8 minutes.
Heat the olive oil in a large heavy-bottomed pot or Dutch oven over medium-high heat. Add the mushrooms, season with salt and pepper, and cook, stirring occasionally until browned, about 5 minutes. Transfer to a medium bowl and set aside.