Finely dice mushrooms or gently pulse in food processor.
In skillet, warm 1 tablespoon olive oil on medium-high heat and add mushrooms, cooking 5-7 minutes, or until golden brown. Remove from heat and cool 5 minutes.
Transfer cooled mushrooms to medium bowl. Add ground beef and salt, mixing until combined. Make four patties.
Add remaining olive oil to pan and cook burger patties on medium-high heat until internal temperature reaches at least 160 F.
Plate and add desired toppings to bun.