Brush bottom and sides of slow cooker insert (4.5 to 6 L capacity) with 1 tbsp (15 mL) butter. Add 1 cup (250 mL) milk. Gradually whisk in the cornstarch until smooth. Whisk in remaining milk and mustard. Stir in garlic, thyme, salt and pepper. Stir in macaroni.
Cover and cook on HIGH 1 hr. 30 min., stirring once halfway through, or until macaroni is tender and sauce is thickened. Stir in cheese. Adjust setting to WARM for serving.
Meanwhile, melt remaining butter in skillet over medium heat. Stir in breadcrumbs. Cook, stirring, 3 to 5 min. until breadcrumbs turn golden. Transfer to serving bowl. Stir in parsley.
Scoop mac and cheese onto serving dishes. Sprinkle with breadcrumbs just before serving.
To make fresh breadcrumbs, cube stale bread and pulse in food processor to coarse crumbs.