To prepare sauce, combine first 4 ingredients; stir well.
To prepare enchiladas, heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion and jalapeño; sauté 5 minutes or until tender, stirring frequently. Add mushrooms; sauté 3 minutes or until tender. Stir in garlic powder, cumin, salt, and spinach; cook 5 minutes or until heated. Combine spinach mixture and 1/4 cup cheese in a large bowl; stir to combine.
Spoon 1/2 cup sauce into a 13 x 9-inch baking dish coated with cooking spray. Warm tortillas according to package directions. Spoon about 1/3 cup spinach mixture down center of each tortilla; roll up. Place filled tortillas seam sides down in the baking dish. Spread remaining sauce evenly over tortillas; top with 3/4 cup cheese.
Bake at 350° for 20 minutes. Top with tomato and green onion.