Melt butter in a large frypan over medium heat. Add carrots, onion, celery and garlic; saute until softened, about 5 minutes. Add mushrooms and cook, stirring frequently, for 8 minutes or until liquid is evaporated. Cool to room temperature.
Preheat oven to 350ºF (180ºC). Line a rimmed baking sheet with nonstick foil.
Pour milk into a shallow bowl. Place bread cubes in milk. Let stand for 2 minutes, stirring occasionally. Squeeze excess milk out of bread cubes, squeezing bread cubes together to form a paste. Discard milk.
Combine vegetable mixture, bread paste and next 11 ingredients (bison through beaten egg) until blended. Shape mixture into a 10x6 inch (25x15 cm) oval loaf and place on prepared pan. Flatten top of loaf slightly. Place bacon slices crosswise over top, tucking in ends underneath.
Bake, uncovered, for 1 1/2 - 1 3/4 hours or until a meat thermometer inserted in centre registers 170ºF (77ºC). Let stand for 10 minute