Preheat oven to 300º. Gather frozen veggie scraps, onions, carrots, celery, mushrooms, and garlic. cut up all the veggies into 1/4 in pieces, and spread out on baking tray lined with parchment paper
In a small bowl, combine miso and olive oil to make a paste. Apply this paste to veggies, making sure to coat evenly. Place tray into the oven and bake for around 1 hour or until veggie mixture turns golden brown in color.
Transfer veggie mixture into a large pot along with 10-12 cups of water and bring to a boil. Once boiling, reduce the heat and let simmer for around 1 hour or until the liquid has reduced by half
Once reduced, strain the mixture through a fine mesh sieve,and add salt to taste. Store this broth in the fridge for up to 2 weeks, and in the freezer for up to 3 months