Preheat the oven: Set oven to 350°F (175°C). Line a baking sheet or loaf pan with parchment paper.
Mix wet ingredients: In a large bowl, whisk egg, milk, and Worcestershire sauce until combined.
Add aromatics: Stir in onion, garlic, salt, pepper, and thyme.
Incorporate breadcrumbs: Add breadcrumbs and mix until evenly distributed.
Add meat: Gently mix in ground beef, avoiding overmixing.
Shape loaf: Form mixture into a loaf on the baking sheet or press into a loaf pan.
Apply glaze: Mix ketchup, brown sugar, and mustard. Spread half over meatloaf.
Bake: Bake for 55-65 minutes, spreading remaining glaze on top during the last 10 minutes. Check internal temperature (160°F for beef).
Rest and serve: Let rest for 10 minutes before slicing. Serve warm.