Cut tomatoes into halves. Place tomatoes, mushrooms and bacon on a baking sheet. Sprinkle vegetables with 1/4 teaspoon salt and pepper and drizzle with olive oil. Place under broiler and cook until bacon is crispy and the tomatoes and mushrooms are browned. Remove from broiler and keep warm.
Combine 4 cups champ, flour, remaining 1/4 teaspoon salt and pepper in a medium bowl.
Heat a well-seasoned cast-iron skillet or nonstick skillet over medium-high heat. Spoon potato mixture into pan and flatten with the back of the spoon, using about 1/8 of the mixture per pancake. Cook until golden brown, about 4 minutes on each side.
To serve, spread butter on potato cakes. Place 1 pancake on each plate. Top with a poached egg, 1 bacon slice, 1 tomato half and 1 mushroom.