Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine the ground beef, 1/2 cup finely diced yellow onion, minced garlic, breadcrumbs, milk, beaten egg, 2 tablespoons chopped fresh parsley, Worcestershire sauce, dried thyme, 1 teaspoon salt, and 1/2 teaspoon black pepper.

Using your hands, gently mix all ingredients until just combined. Be careful not to overmix, as this can result in a tough meatloaf.

Transfer the meat mixture to a 9x5 inch loaf pan. Press firmly and shape into an even loaf. If desired, spread 2 tablespoons of ketchup evenly over the top for a classic glaze.

Place the loaf pan in the preheated oven and bake for 60-70 minutes, or until the internal temperature reaches 160°F (71°C) when a meat thermometer is inserted into the center.

Once baked, remove the meatloaf from the oven and let it rest in the pan for 10-15 minutes. This allows the juices to redistribute, ensuring a moist meatloaf. Carefully drain any excess fat before slicing.

While the meatloaf rests, heat 1 tablespoon olive oil in a large skillet over medium heat. Add the sliced cremini mushrooms and 1/4 cup finely diced yellow onion. Sauté for 5-7 minutes, until the mushrooms are softened and browned, and the onion is translucent.

Sprinkle 2 tablespoons all-purpose flour over the mushrooms and onions. Stir constantly for 1-2 minutes to cook out the raw flour taste, forming a roux.

Gradually whisk in 2 cups of beef broth, stirring continuously to prevent lumps. Bring the gravy to a simmer, then reduce heat to low and cook for 5-10 minutes, or until the gravy has thickened to your desired consistency. Season with salt and pepper to taste.

Slice the rested meatloaf and serve warm, topped generously with the homemade mushroom gravy. Garnish with additional chopped fresh parsley, if desired.