Cook spinach in large saucepan over medium heat.
While spinach is cooking, combine garlic, onion, and mushrooms in food processor. Using pulse button, process until vegetables are about 1.8 to 1.4 inch in size; set aside.
Melt butter in nonstick pan over medium-high heat. Add onion mixture and saute until onion is translucent. Remove from heat; set aside.
When spinach is done, drain and squeeze out excess water.
Place bread in food processor and process to make crumbs. Add spinach, salt, white pepper, nutmeg, chili powder, and eggs. Process until thoroughly blended.
Add reserved onion mixture and process to mix. Transfer to mixing bowl and stir in 6 tablespoons mixed cheeses.
Spoon portion of filling into tortilla, roll up, and place in 13*9*2 inch casserole dish, seam side down. Pour Cilantro Cream Sauce over top and sprinkle liberally with grated cheeses. Heat under broiler until cheese melts and filling is thoroughly heated.
Cilantro Cream Sauce:.
In 2 quart saucepan, combine cream, cayenne, salt, and a little of the cornstarch/water mixture. Heat, stirring, over medium heat until thickened; sauce should be creamy, but not runny. Add cornstarch mixture, a little at a time, cooking well after each addition to reach desired consistency. Stir in cilantro. If made ahead, reheat before using.