In a large skillet, melt the butter over moderate heat. Add the onion, green pepper, celery, and thyme. Reduce the heat to low; cook covered, stirring one or twice, until the vegetables are very soft (about 20 minutes). Remove from heat; cool to room temperature.
Preheat oven to 350°F In a large bowl, combine cooked vegetables and beef. Add oats, 1/3 cup ketchup, eggs, salt, and pepper; mix well. Spoon the meat mixture into a large shallow baking dish. Form the meat into a flat loaf about 2 inches high and smooth the top with the back of a spoon. Spread the remaining 1/3 cup ketchup over the loaf; arrange the bacon strips on top.
Bake the meatloaf for 1 hour or until it is just cooked through and shows no trace of pink. (A thermometer will register 145°F in the center of the loaf). Let the meatloaf stand for about 10 minutes before slicing and serving.
NOTES: For my large family, I double the recipe and increase cooking time to approximately 1-1/2 hours. A dash of Worcestershire sauce or Pick-A-Pepa kicks up the flavor nicely. You can also substitute turkey bacon, or omit the bacon and add a dash of liquid smoke. Using tomato paste on top of the meatloaf instead of ketchup adds a tangy flavor.