Preheat oven to 400 degrees F. Line a sheet pan with foil and spray with cooking spray. Set aside.

To prepare the mushrooms, remove the stems and gently use a small spoon to remove the gills, being careful not to break the caps. Gently clean the caps with a damp paper towel.

In a small bowl, mix together the minced ginger, minced garlic, soy sauce, sesame oil, and brown sugar.

Place the mushrooms, cavity side up on the prepared sheet pan. Brush the insides with the soy sauce mixture. Roast in the oven for 15-20 minutes or until the mushrooms are caramelized and cooked through. Remove from the oven and let cool on the sheet pan.

Prepare the tempura batter using the tempura mix and iced water according to the package directions. Place in a bowl for dredging.

Place the panko breadcrumbs in another bowl for dredging.

Line another sheet pan with parchment paper. Dip the mushrooms in the tempura batter and then in the panko breadcrumbs, pressing the breadcrumbs to make sure they stick. Place on the prepared sheet pan. Place the breaded mushrooms in the refrigerator until ready to fry.

Preheat oil to 350 degrees F. Fry the portabella mushrooms until golden brown, about 2-3 minutes per side. Drain on a wire rack. Slice into ½” slices and serve with tonkatsu sauce.