- With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
- In a large non-stick skillet over medium-high heat, brown the mushrooms in half of the oil. Season with salt and pepper. Set aside on a plate.
- In the same skillet, brown the onions in 1 tbsp (15 ml) of the oil. Season with salt and pepper. Set aside with the mushrooms.
- In the same skillet, brown half of the meat at a time in the remaining oil. Season with salt. If desired, thinly slice the meat.
- Cut the buns in half horizontally and place on the baking sheet. Fill the buns with the meat, mushrooms and onions. Sprinkle with the cheese and bake for a few minutes or until the cheese has melted.
- Meanwhile, in a bowl, whisk together all of the ingredients. Season with salt and pepper.
- Cover the open subs with the lettuce and tomato slices. Drizzle with the dressing and serve immediately.