Preheat oven to 450 degree F. Line a 15x10x1-inch baking pan with foil, and oil the foil well; set aside.
Wash asparagus and break off woody bases where spears snap easily. Scrape off scales, if desired. Place tomato halves, cut sides up, and asparagus spears on prepared pan. Drizzle with the olive oil; sprinkle lightly with salt and pepper. Roast, uncovered, for 8 to 15 minutes or until asparagus is crisp-tender and tomatoes are heated through.
Meanwhile, for Rarebit Sauce, in a medium saucepan heat milk and margarine or butter over medium heat. Gradually add hot milk mixture into beaten egg yolks, stirring constantly; stir in mustard and red pepper. Return mixture to saucepan. Continue to heat, stirring constantly, until mixture coats a metal spoon. Stir in cheese, a handful at a time, stirring until melted after each addition. when all cheese has been added, continue to heat for 1 to 2 minutes more or until smooth, stirring constantly; do not boil.
Toast the bread; rub one surface of each slice with the cut side of a garlic clove, Transfer toast to serving plates, garlic side up. Top each toast slice with 4 tomato halves and several of the asparagus spears. Spoon hot rarebit sauce over tomatoes and top with a sprig of parsley. Serve immediately. Makes 6 main-dish servings.