Combine Topping ingredients in small bowl; mix to combine then set aside until ready to use.
Heat oven to 400 degrees, then butter the bottom of a 9X13 baking dish.
Cut fillets into 4 portions and place in baking dish; sprinkle with a little salt and pepper.
Heat a non-stick skillet to medium. Add 1 Tb. butter and 1 Tb. olive oil. When warm, add chopped mushrooms and leeks and sprinkle with 1/4 tsp. salt and 1/8 tsp. pepper. Cook until soft (about 5 minutes) and stir frequently. Add spinach and combine the leaves with the warm mixture so they wilt. When the spinach looks soft (about 1 minute), remove from heat.
Top each fillet with some of the vegetable mixture, then sprinkle each with Panko mixture. Melt the butter, then sprinkle a little over each bundle.
Bake about 15 minutes (depending on thickness of fillets). Check around the 10 minute mark to ensure the coating is browning, but not burning. The fish will be bubbly, bright white and flaky when cooked. If it's still opaque, leave it in a bit longer.