Preheat oven to 325°F.
Use a small biscuit cutter or a small cup/glass to cut sandwich bread into circles to fit in the bottom of a 6-cup muffin tin.
Over medium-low heat, cook the onions, olive oil, salt, pepper, and thyme until onions are translucent but not browned, approximately 15 minutes.
Add the Worcestershire sauce, chicken stock, and tomato paste. Mix well. Allow mixture to cool to room temperature.
In a large bowl, combine the bread crumbs, egg, and onion mixture. Add ground turkey and mix until completely incorporated.
Press turkey into muffin cups. Spread ketchup on top. Bake until the internal temperature reaches 165°F and the meatloaf is cooked through. (This takes about 1 - 1 1/2 hours for meatloaf in a loaf pan and much less time, about 35 minutes or so, for the muffins).
Let meatloaf muffins stand 10 minutes, then carefully remove them from tin, gently removing and discarding the soggy bread from the bottom.