The vegetables for this colorful, vitamin-packed kugel can be prepared quickly in the food processor. Double the recipe for a large crowd. It's a winner! Use frozen spinach instead of fresh if you prefer.
Preheat oven to 350 degrees F. Wash spinach thoroughly. Remove and discard tough stems. Cook spinach in a covered saucepan until wilted, about 3 minutes (or microwave on HIGH for 4 minutes). Don't add any water. The water clinging to the leaves will provide enough steam to cook it. Cool and squeeze dry.
Heat oil in a non-stick skillet on medium heat. Saute onion, celery, red pepper and carrots for 5 minutes, until golden. Add mushrooms and cook 5 minutes longer (or cook vegetables uncovered in the microwave for 6 to 8 minutes on HIGH).
Chop spinach coarsely. Combine with remaining ingredients and mix well. Pour into a sprayed 7" x 11" Pyrex casserole. Bake uncovered at 350F for 45 to 50 minutes, until firm. Cut into squares to serve.