Preheat oven to 400 degrees.
Whisk together sake, maple syrup, garlic, ponzu, 1 T lemon juice and ginger until smooth. (I like to measure the ponzu sauce into a liquid measuring cup and add the rest of the ingredients to the cup.)
Put salmon into a large ziplock bag. Pour mixture into bag. Seal and place in refrigerator for 30 minutes.
Put spinach in a large bowl.
Toss together asparagus and oil and place on a baking sheet. Spread in a single even layer. Roast in oven until the asparagus is just tender when poked with a fork, about 8 minutes. Remove asparagus from oven and allow to cool slightly. Toss the asparagus with the spinach. The warm asparagus will wilt the spinach.
Transfer salmon, skin side down, to the baking sheet, reserving 2 T of the marinade. Roast until salmon flakes easily (or your preferred doneness), 12 – 15 minutes, brushing with the marinade halfway through the cooking process. Remove from oven.
While salmon is cooking, toss the spinach and asparagus again and then once more with the balsamic vinegar, salt, pepper and remaining lemon juice.
Remove salmon skin by gently sliding salmon off of the skin with a spatula and serve salmon on top of a bed of the spinach and asparagus. Garnish with lemon wedges.