Cook eggs in medium nonstick skillet on medium heat 2 to 3 min. or until set, stirring occasionally. Remove from heat.
Cover bottom halves of muffins with Singles, ham, eggs, spinach and tomatoes.
Spread mayonnaise onto split sides of remaining muffin halves; place, mayonnaise sides down, over sandwiches. Wrap individually in waxed paper or foil.
Refrigerate until ready to serve.
Place 1 unwrapped sandwich on microwaveable plate just before serving. Microwave on HIGH 1 min. or until heated through. Repeat with remaining sandwiches if necessary for additional servings.