Preheat oven to 200°C. Cook the pasta in a large saucepan of boiling water following packet directions or until al dente. Drain well.
Heat a large saucepan over medium heat. Add the bacon and cook, stirring, for 3 mins or until golden brown. Use a slotted spoon to transfer to a heatproof bowl. Add butter to the pan. Cook for 1 min or until butter melts. Add flour and mustard powder. Cook, stirring, for 1 min or until mixture bubbles and is grainy. Gradually add milk, whisking constantly until combined. Cook, stirring, for 2 mins or until mixture comes to the boil. Add the beer or stock and cook, whisking, for 3-4 mins or until the sauce boils and thickens. Season. Remove from heat. Stir in 1 cup (120g) cheddar. Stir in the bacon, chives and pasta. Season.
Transfer the pasta mixture to a 6-cup (1.5L) ovenproof dish. Sprinkle with the remaining cheddar. Bake for 15 mins or until golden brown.
Meanwhile, line a baking tray with baking paper. Arrange the bread pieces in a single layer on the lined tray. Spray with olive oil spray. Bake for 8-10 mins or until golden.
While pasta mixture is baking, heat oil in a small frying pan over medium heat. Add the sage and cook, turning, for 1-2 mins or until crisp. Transfer to a plate lined with paper towel.
Top the pasta mixture with the bread pieces and fried sage to serve.