Heat the Instant Pot insert on the sauté setting and add the 1 tablespoon of avocado oil.
Add the London broil and brown on each side until a crust forms, about 1 minute per side. Turn the sauté setting off.
Add the remaining ingredients (except mushrooms, salt and remaining avocado oil).
Pressure cook on high for 3 minutes. Let the pressure release naturally for 7 minutes, then release any remaining pressure.
While the meat cooks, heat a skillet over medium heat and add the oil.
Add the mushrooms and salt and cook, stirring regularly, until mushrooms are browned and have released their liquid (softened).
Remove the London broil from the instant pot and let rest for 10 minutes.
Turn the Instant Pot to the sauté setting and simmer the sauce. Thicken with a cornstarch slurry if desired (1 tablespoon cornstarch mixed with 1 tablespoon water and added to the liquid while whisking).
Slice the London broil and serve with the sautéed mushrooms and cooking liquid.