Heat the oil in a large skillet, add the onion and mushrooms and cook until the liquid is evaporated and the mushrooms begin to brown about 8 minutes. Stir in the carrots, tomato paste, thyme and garlic and cook, stirring, 2 minutes more. Allow to cool completely.
In a large bowl combine the beef, oats, eggs, Worcestershire sauce, mushroom mixture, salt and pepper. Mix until just well combined. Transfer the mixture to a 9x13 inch baking dish and shape into a loaf about 5 inches wide and 2 inches high.
In a small bowl whisk the tomato sauce, molasses and mustard and pour over the meatloaf.
Cook the meatloaf until a meat thermometer registers 160 degrees, about 55-60 minutes. Remove from the oven and let rest 15 minutes before slicing.