Preheat oven to 350 F.
In a small bowl, stir yogurt into bread crumbs / cubes until well covered. Let sit for 15 minutes.
MEANWHILE, mix together the ground beef, mushrooms, egg, garlic powder, salt, brown sugar, vinegar, and red pepper flakes in a large bowl. (A stand mixer works well here)
Mix the bread-yogurt mixture into the meatloaf mixture. Stir thoroughly.
Place meatloaf mixture into GLASS meatloaf pan (9x5").
Bake for 75-85 minutes or until internal temperature is at least 165 degrees F.
Remove from oven and let cool for 5-10 minutes.
Carefully drain most (not all) of the fat into a can or other metal container. (discard the fat after it cools).
Slice meatloaf and serve hot with a side salad, pickled red onions, and additional marinated mushrooms.