HEAT an extra-large non-stick frying pan over medium-high. Add bacon. Cook, stirring occasionally, until crisp, 7 to 8 min. Transfer bacon to paper towel-lined plate. Discard fat, leaving 1 tbsp in pan. Add onions and 1/4 tsp salt. Season with fresh pepper. Cook, stirring often, until soft and browned, 10 to 12 min. Stir in bacon.
WHISK canola oil with cider vinegar, honey and 1/4 tsp salt in a large bowl. Add torn lettuce, sliced apple and chopped walnuts. Toss to coat.
HEAT 3 cups reserved mash (from Cottage Pie with Celery Root Mash) with milk in a medium pot over medium, stirring often, until warmed through, 5 to 6 min. Divide among 4 plates. Top with onion mixture. Serve with salad.