Combine sausage, onion, tomato, bell pepper, garlic, mushroom and oregano, in a bowl. Season with salt and pepper to taste. Set aside.
Preheat oven to 400 degrees F. Take 4 heavy-duty aluminum foils (about 16 inches length). Divide the sausage mixture evenly in the center of each foil sheet.
Place 3 butter cubes on each one. Bring up foil sides and fold to seal, leaving room for heat circulation inside. Place them on the oven rack.
Bake in the oven for 30 minutes.