Cider Dijon Sauce: In saucepan, heat oil over medium heat; cook onion and apple, stirring occasionally, until tender and light golden, 10 to 12 minutes. Stir in cider, broth, molasses and Dijon mustard; bring to boil over high heat. Reduce heat and simmer, stirring occasionally, for 15 minutes. Let cool slightly. In blender, purée sauce until smooth.
Return sauce to pan; bring to boil. Whisk in grainy mustard, salt and pepper. Whisk cornstarch with 3 tbsp water; whisk into sauce. Bring to boil; cook until thickened, 1 to 2 minutes. (Make-ahead: Let cool for 30 minutes; refrigerate in airtight container for up to 5 days.)
Cabbage Rolls: In saucepan of boiling salted water, cook carrots and potatoes until fork-tender, 15 to 18 minutes; drain well.
In same pan, melt butter over medium heat; cook garlic and red onion, stirring occasionally, until lightly browned, about 3 minutes. Sprinkle in thyme, caraway seeds, salt and pepper. Turn off heat; return carrots and potatoes to pan. Mash until smooth; stir in milk. Fold in corned beef. (Make-ahead: Let cool for 30 minutes; refrigerate in airtight container for up to 2 days.)
Working in batches, in saucepan of boiling water, cook cabbage leaves until slightly softened; rinse to cool. Using knife, cut triangle from bottom of each cabbage leaf to remove thick white portion. Divide potato filling among leaves. Tuck in sides and roll up.
Assembly: Spoon one-third of the Cider Dijon Sauce over bottom of 13- x 9-inch (3 L) baking dish. Place cabbage rolls, seam side down, on top of sauce. Spoon 1 cup of the sauce over cabbage rolls. Cover with parchment paper; seal pan with foil. Bake in 350°F oven until filling is warm and sauce is bubbly, 45 to 55 minutes. Serve with remaining sauce.
Makes 18 to 20 rolls.