Preheat the oven to 425ºF. Rinse the portobello mushrooms and then slice into about ¼-inch slices. Add this to a large flat container. In a bowl, combine the soy sauce, balsamic vinegar, garlic powder, and chili flakes, then pour over the sliced mushrooms. Turn the mushrooms gently with your hands or chopsticks to coat each slice with the marinade. You can also use a pastry brush to brush the mushroom slices with the marinade. Set aside to marinate for 15 minutes.

Arrange mushrooms in a single layer on a lined baking sheet. Bake at 425ºF for about 12 minutes or until cooked to your desired doneness. Be aware that cooking too long will start to burn the sugars in the balsamic vinegar!

Meanwhile, prepare your bread! I recommend slicing open your bun and crisping the inside. Bring a skillet to medium-high heat. When hot, add a small drizzle of olive oil and then place the bun cut-side down in the skillet. If you have a stiff or crusty bread, I recommend covering with a lid to help it soften. Cook for about 3-5 minutes or until the bun is nicely grilled on one side. Remove from heat. You can do this all at once or in two batches if your pan isn’t large enough.

Prepare your sandwich! On one side of the bread, spread the pesto and sprinkle with about ½ a Tablespoon of hemp hearts. On the other side, spread with goat cheese - I like for the cheese to get a little melty. Next add the mushrooms atop the pesto and hemp hearts, and the red bell pepper strips to the goat cheese.

Finally, add a handful of fresh arugula and close your sandwich and enjoy!