Step 1Heat the oven to 400 degrees.
Step 2Lightly spray the potatoes with cooking spray. Bake the potatoes until they're tender (test them with the sharp point of a paring knife), about 1 hour. Alternatively, cook them in a microwave oven on high power for 6 minutes, then bake them in a 400-degree oven until they're tender and the skins are crisp, about 15 minutes more. Reduce the heat to 375 degrees.
Step 3When the potatoes are cool enough to handle, split them lengthwise and scoop out the flesh, leaving about a 1/4-inch shell. Place the scooped potato in a bowl.
Step 4Meanwhile, melt the butter in a small nonstick skillet over medium heat. When it's hot, sti r in the garlic and green onion. Remove the skillet from the heat. Transfer the butter to the potatoes along with the wasabi paste, sour cream, salt, pepper to taste and 1/2 cup cheese. Use a potato masher or fork to mash the ingredients together; keep the mixture slightly lumpy. Taste; adjust the seasoning.
Step 5Divide the mixture among the 4 potato shells, mounding it slightly. Sprinkle the tops evenly with the remaining cheese. Use your hands to gently press the cheese into place. Place the potatoes in a single layer on a baking sheet. (They can be made a few hours ahead up to this point and kept at room temperature.)
Step 6Bake until tops are just lightly browned, about 25 minutes. Serve hot.