Preheat the oven to 375°F Lightly oil a 9-by-13 inch flameproof baking pan. In a large bowl, combine the onion, oats, the 1/2 cup tomato sauce, the eggs, Worcestershire sauce, 1 teaspoons salt, and 1/4 teaspoons pepper. Stir to mix thoroughly. Add the ground meats and mix well with clean hands. Rinse an 8 1/2-by-4 1/2-inch loaf pan with water and pack the meat mixture into the wet pan. Unmold the meat mixture into the prepared baking pan.
Bake for 1 hour. Spread the 3 tablespoons tomato sauce over the top of the meat loaf and continue baking until an instant-read thermometer inserted in the center of the loaf registers 165F, about 10 minutes longer.
Meanwhile, melt the 1 tablespoon butter in a skillet over medium-high heat. Add the mushrooms and cook, uncovered, stirring from time to time, until lightly browned, about 8 minutes, set aside.
Using a large spatula, transfer the meat loaf to a platter and cover loosely with aluminum foil. Spoon off the clear yellow fat from the baking pan and measure it. Add melted butter as needed to make 2 tablespoons. Return the 2 tablespoons fat to the meat juice in the baking pan and place on the stove top over medium-low heat. Whisk in the flour until smooth to make a roux. Let bubble for 1 minute. Whisk in the stock, then scrape up the browned bits from the pan bottom. Bring to a simmer over medium-high heat, whisking often. Reduce the heat to medium-low add the reserved mushrooms, and simmer until lightly thickened, about 5 minutes Season to taste with salt and pepper.
Slice the meat loaf and serve hot, garnished with thyme and drizzled with gravy.