Pre-heat oven to 450 degrees. Line a cookie sheet with aluminum foil. Lightly brush each red pepper with a small amount of vegetable oil and season mushroom with salt and pepper. Place both peppers and the portabella mushroom in the oven for 15 minutes. Remove portabella mushroom from the oven, and turn the peppers with tongs. Bake the peppers for another 15 minutes, rotate again, and bake for another final 15 minutes or until fully roasted.
Remove the peppers from the oven and immediately place them in a large ziploc bag. Let cool for 15-20 minutes. Remove peppers from bag, you should be able to peel the skin off each pepper with your fingers and a knife. Slice into thin strips. Also slice the portabella into thin strips. Butter one side of each slice of bread. Assemble sandwich by adding your desired amount of portabella strips and then roasted red pepper strips. Sprinkle with salt, pepper and garlic powder. Top with the sliced gouda cheese. Use a panini press or skillet, and cook until the outside is toasted and the cheese is melted. Enjoy!