In large microwavable bowl, place cheese cubes, butter & milk. Microwave uncovered on High 8 - 10 minutes. Stir with whisk; microwave on High 4 minutes longer. Stir again; repeat microwaving until all cheese is melted & mixture can be stirred smooth, a total of about 15 minutes.
Preheat oven to 350 degrees F. Cook macaroni in boiling salted water 3 minutes less than directed on box (macaroni should be firm). Drain; place in 15 x 10" (4 qt) glass baking dish. Pour melted cheese sauce over macaroni; mix well. Stir in shredded cheese, tomatoes & spinach.
Cover dish with foil; bake 30 minutes. Uncover; sprinkle bacon & bread crumbs evenly over top. Bake 5 minutes longer or until top is lightly browned. Cool at least 10 minutes before serving.
NOTES: A little chopped sautéed onion can be added for variety along with a little ground mustard.
The cheese sauce can also be melted in a saucepan on the stove in place of the microwave.