Slice the baguette in half lengthwise, split open, and toast in the preheated oven until lightly browned, 7 to 9 minutes.
Heat the olive oil in a skillet over medium heat, and cook and stir the mushrooms and artichoke hearts until the mushrooms have given up their liquid and have started to brown, about 10 minutes. Stir in the Parmesan cheese, garlic and onion seasoning, and salt and pepper, and cook and stir until the mixture has thickened, about 5 more minutes.
Fill the toasted bread with the mushroom filling, close the sandwich, cut in two, and serve.
