Preheat oven to 425° F. Roughly chop mushrooms and toss in olive oil. Spread in an even layer on a sheet pan and roast in the oven for about 20 minutes, until soft. Drain off excess liquid.
Place mushrooms in food processor and process until it reaches the consistency of ground beef.
Place ground beef, mushroom mixture, onion, egg, breadcrumbs, salt, pepper, and thyme into a large bowl. Mix with a spatula until well combined.
Reduce heat to 350° F. Place meatloaf mixture into a loaf pan and bake for 35 minutes. Remove from oven, spread ketchup over top of meatloaf and return to oven. Bake for an additional 15-20 minutes, or until internal temperature reaches 160° F. Let stand for 10 minutes before serving.
