1. Place spinach in 1-quart casserole. Cover tightly and microwave on high (100%) 4 minutes; stir. Cover and microwave until tender, 2 to 3 minutes longer; drain.
2. Beat egg, flour, salt, lemon juice and pepper until smooth. Stir egg mixture and onion into spinach. Cover tightly and microwave on medium-high (70%) 3 minutes; stir. Microwave until hot, 2 to 3 minutes longer. Sprinkle with peanuts. Let stand 5 minutes.