1. SCRAPE the underside of the mushroom to remove the black gills with a teaspoon. Discard the gills.
2. HEAT a griddle or large skillet over high heat. Add the oil, mushroom, and onion. Cook for 8 to 10 minutes, stirring and turning occasionally, or until the onion begins to brown and the mushroom is cooked through. Add 1 tablespoon water if the onion begins to stick.
3. SPREAD the yogurt, mayo, and relish on 1 side of the baguette and top with the pork and ham. Spoon the onion on top. Cut the mushroom in half and layer it on top of the onion.
4. TOP with the remaining piece of baguette and return the sandwich to the griddle or skillet.
5. PLACE a sandwich press or another heavy pot on top to compress the sandwich. Remove the sandwich press or heavy pot and cook over low heat for 1 minute total, turning the sandwich once, or until it browns lightly. Serve immediately or wrap in aluminum foil and refrigerate until ready to serve.