Serve these open-face sandwiches with Quick Dill Pickles and Caramelized Sweet Potato Wedges.
Heat broiler. Put mushrooms in a single layer on a baking pan, season with salt and pepper, and drizzle lightly with olive oil. Place pan under broiler until mushrooms brown around edges, 2 to 4 minutes. Transfer mushrooms to a large bowl; set aside. Using same pan, repeat process with tomatoes until they are soft, 3 to 4 minutes. Repeat with onions, until they are tender, 6 to 8 minutes. Repeat with peppers, until skins start to blister, 6 to 8 minutes. When peppers are cool enough to handle, remove skin and stems; cut into 1-by-2-inch pieces. Combine all vegetables in a bowl, and toss with parsley. Season with salt and pepper, and set aside.
Halve bread lengthwise. Spread mustard on both sides; layer with roasted vegetables. Arrange turkey and/or ham over vegetables; top with cheese. Place on baking pan; transfer to broiler until warmed through and cheese is melted, about 2 minutes. Serve.