Cut bread loaf in half lengthwise. Scoop out bread, leaving 1/4-inch-thick shells. Tear reserved bread into pieces, and measure 1 1/2 cups, reserving remaining bread pieces for other use. Set bread shells and 1 1/2 cups breadcrumbs aside.
Melt 1 1/2 tablespoons butter in a large skillet over medium heat; add chopped onion and mushrooms, and saute 8 minutes or until tender.
Stir together egg and 2 tablespoons ketchup in a large bowl. Add onion mixture, 1 1/2 cups breadcrumbs, 1/2 cup cheese, ground beef, and next 3 ingredients, blending well. Shape mixture into a 6- to 7-inch loaf. Place on a lightly greased rack in a roasting pan.
Bake at 350° for 1 hour or until done.
Spread bottom bread shell with mayonnaise; top with meat loaf. Top with remaining ketchup; sprinkle with remaining 1 cup cheese. Top with remaining bread half. Melt remaining 1 tablespoon butter; brush over bread top. Wrap in aluminum foil.
Bake at 350° for 10 to 15 minutes or until heated.