"After tinkering for a few years to recreate childhood memories of truly yummy, better-than-steak, "more, please" meatloaf, I finally landed on this and can stop tinkering! This is not a tomato-ey or tangy or anything meatloaf - just savory hearty beef flavor."
Heat the oil in a skillet over medium heat, and cook the mushrooms a few minutes until they start to release liquid. Add onion and pepper and cook about 5 minutes, until onion is tender. Add garlic and cook another 5 minutes or so, being careful not to brown or burn garlic. Vegetables should be very well cooked and barely recognizable. Remove from heat, and season with salt and pepper.
While vegetables are cooking you can crush crackers and toast/crumble the bread.
In a large bowl, mix the vegetables and beef, crumbled bread, crushed crackers, egg, sour cream, Worcestershire sauce, ketchup and mustard. Gradually stir in the milk 1 tablespoon at a time until mixture is moist, but not soggy. Transfer the mixture to a 5x9 inch loaf pan.
Bake uncovered in the preheated oven 40 minutes. Increase oven temperature to 400 degrees F (200 degrees C), and continue baking 15 minutes, to an internal temperature of 160 degrees F (70 degrees C).