Preheat the oven to 350° F.
Prepare the onions: Keeping the outer layers on the onions, remove the stem and using a paring knife, make 6 vertical cuts around the onion, 2 layers deep, and pull down the 6 sections forming a crown on top.
In a bowl, mix the filling ingredients except for the mushrooms. Divide the filling ingredients in the center of four 12-inch-by-12-inch pieces of aluminum foil.
Hold the crown of each onion and place on top of the filling ingredients. Draw the squares of aluminum foil from 4 corners to the top center like a pyramid with the crown sticking out. Seal well by pressing in the foil.
Place the onions on a sheet pan and bake in the oven for about 1 hour and 10 minutes, or until soft.
Make the sauce: In a small, hot skillet, heat the olive oil and sauté the mushrooms until soft. Add the cooking juices from the onions in the aluminum foil and the contents of the onion pouch (but not the onions themselves) and gently heat with the ham and tomato. Season to taste with salt and pepper.
To serve, spoon the sauce up to the rim of a dinner plate. Place the onion in the center, and garnish with fresh chopped chives.Makes 4 Source: