Cook cauliflower just until tender; do not overcook. Drain it and keep it hot. Cook noodles. Drain, mix with parsley, margarine and salt. Turn into baking dish and top with cauliflower. Combine mayonnaise, onion and lemon juice and fold into whipped egg white. Spread on top of cauliflower. Place under broiler for 3 to 4 minutes, until puffed and brown.