Melt 2 tablespoons butter in large skillet over medium-high heat. Add leeks, shiitake mushrooms, and red pepper, saute 3 to 4 minutes. Stir in spinach. Cook 2 to 3 minutes or until all liquid is evaporated; cool to room temperature. Stir in cheddar cheese and 1/2 cup of Parmesan cheese.
In large mixing bowl whisk eggs, mustard, cream or half-and-half, salt and pepper.
Spread spinach mixture in bottom of greased 3-quart baking dish. Pour egg mixture over spinach. (Can be made a day ahead; refrigerate, covered.)
Before baking, melt remaining 4 tablespoons butter. Combine with panko, remaining 1/2 cup of Parmesan cheese and salt and pepper to taste. Sprinkle over egg mixture.
Bake uncovered in preheated 350 F oven 35 to 40 minutes or until knife inserted in center comes out clean.