Melt 2 tablespoons butter in large skillet over medium-high heat. Add leeks, shiitake mushrooms, and red pepper, sauté 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, Caramelized Onion 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 degree F. oven for 35 to 40 minutes or until knife inserted in center comes out clean.