Preheat the oven to 400 degrees - use a convection setting if possible. Line a baking sheet with tin foil then coat with olive oil cooking spray.
Combine the egg and milk together with some sea salt and freshly cracked pepper, to taste. Pour the panko crumbs onto a large plate. Dip the mushroom into the egg mixture then into the panko crumb mixture. Coat both sides evenly with the crumbs. Place the mushroom onto the lined baking sheet stem side up. Repeat with the remaining mushrooms.
Place the mushrooms into the oven to bake for 15-20 minutes.
While the mushrooms are baking. Heat the olive oil into a large pan over medium heat. Add the garlic slivers and cook, stirring constantly, for 1 minute. Add the kale and crushed red pepper flakes then cook, stirring constantly for 30 seconds. Add the broth to the pan then cook, stirring occasionally, for 7-10 minutes. Season with sea salt and freshly cracked pepper, to taste.
Remove the mushrooms from the oven. Place a dollop of cheese into each center (bowl) of the mushrooms. Place back into the oven and cook for 7-10 minutes or until mushrooms are done and cheese is melted. Remove from the oven; set aside. Toast the buns in the oven, if desired.
Place the mushroom on the toasted bun then add some drained kale and an heirloom tomato slice followed by the other bun. Serve immediately. Enjoy!