Heat a large wide-base pot over medium heat and add the olive oil. Once the pan is hot, add the mushrooms. Cook, stirring as infrequently as possible, until lightly golden and any water has evaporated. Do this in batches if necessary.
Stir in the carrot and onion, then add the water, herbs, peppercorns, and salt.
1 large carrot, 1 small onion, 2 litres water, 1 small handful thyme, 1 bay leaf, 4 black peppercorns, 2 teaspoons sea salt
Cover and increase the heat to bring the stock to a boil. Once it reaches a boil, reduce the heat and let simmer, covered, for one hour.
After simmering, the vegetables should be well cooked and the broth should be cloudy and golden. Strain the broth into another container, like another pot, through a fine mesh sieve and use or discard the vegetables.
Taste and season to your taste. Transfer the broth into container(s) and use or store as needed.
Notes
When I make this broth, I do the cremini mushrooms first, then remove and set aside while I cook the other mushrooms. Add them back in with the onion and carrot if you are also using a smaller pot.