In a large enameled cast-iron casserole, heat the olive oil until shimmering. Add the chuck and sirloin and cook over moderately high heat, breaking up the meat into large chunks, until no pink remains. Add the garlic, oregano and crushed red pepper and cook until fragrant. Stir in the tomato paste and cook until the meat is coated. Add the tomatoes and their juices and the tomato puree along with the chicken stock, bay leaves, thyme and sugar. Season with salt and pepper and bring to a boil. Simmer over moderate heat, stirring occasionally, until thickened and reduced to 8 cups, about 1 1/2 hours. Remove the bay leaves and thyme sprigs.