With the rack in the middle position, preheat the oven to 190°C (375°F).
On a work surface, butterfly the chicken: using a chef’s knife or kitchen shears, cut out the backbone of the chicken. Open the chicken and spread it out flat. Transfer it to a baking sheet, breast-side up. Cover with the butter and Dijon mustard. Sprinkle with the breadcrumbs and paprika. Season with salt and pepper.
Bake for 50 minutes for small chickens and 1 hour and 15 minutes for a large chicken.
Meanwhile, in a skillet over medium-high heat, brown the bacon. Add the mushrooms and sauté until golden brown. Remove from the heat and add the tomatoes. Season with salt and pepper.
Add the mushroom mixture around the chicken and cook for an additional 15 minutes or until a meat thermometer inserted into the thigh, without touching bone, reads 74°C (165°F).
On a work surface, cut the chicken into pieces to separate the breasts and thighs. Serve with the mushroom mixture and rice or mashed potatoes, if desired.