Preheat the oven to 375°F and line a baking sheet with parchment paper.
Heat a skillet over medium heat and cook the mushrooms and onion for 6 to 8 minutes until the liquid evaporates and the mixture turns golden. Add the garlic and cook for 1 minute more, then remove from heat and let cool slightly.
In a large bowl, combine the ground beef, cooled mushroom mixture, breadcrumbs, egg, thyme, salt, and pepper. Mix with your hands until just combined.
Divide the mixture into 8 equal portions and shape each into a small loaf. Place the loaves on the prepared baking sheet.
Stir the ketchup and brown sugar together in a small bowl. Spread the glaze evenly over the top of each mini meatloaf.
Bake for 25 to 30 minutes until the internal temperature reaches 160°F. Let rest for 5 minutes before serving.