Melt 2 Tablespoons butter in large skillet over medium-high heat. Add leeks, shiitake mushrooms, and red pepper, sauté for 3-4 minutes. Stir in spinach. Cook 2-3 minutes or until all liquid is evaporated, then 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 if refrigerated and covered.)
Before baking, melt remaining 4 Tablespoons of 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 35-40 minutes, or until knife inserted in center comes out clean.