Preheat the oven to 375 degrees and lightly grease a loaf pan.
Heat the olive oil in a skillet over medium heat. Add the diced onion and chopped mushrooms. Cook for 6 to 8 minutes until the vegetables soften and most of the liquid evaporates. Stir in the minced garlic and cook 1 minute more. Remove from heat and let cool slightly.
In a large bowl, combine the ground beef, flaxseed meal, egg, dried thyme, salt, and black pepper. Add the cooled mushroom and onion mixture.
Mix everything with your hands until evenly combined. Transfer to the prepared loaf pan and press into an even layer.
Bake for 45 to 50 minutes until the internal temperature reaches 160 degrees. Let the meatloaf rest 10 minutes before slicing.